I've played the raw blu-ray video files on my computer and I can assure you that the excellent SD video quality of the Season 2 episodes isn't from my player or TV. Turbine's video quality is just that good. :-)
Why are the blu-ray files so good while the streaming versions are so terrible?
My theory: Turbine Media Group acquired copies of the master tapes from NBCUniversal and created their own digital files from the tapes. I have never seen The Hub's broadcasts, but if the quality was superior to streaming services and the DVDs, they may have done the same.
Bargain Bin Operation: Mill Creek said they didn't have the resources to do their own scans of the master tapes, but Mill Creek is a low budget operation. They cram whatever files they get onto as few discs as possible for the lowest price point possible. As you said, their SLIDERS release was for people who vaguely remembered the show and would pay the discounted $20.
Even Mill Creek's blu-ray releases, described as adequate in various reviews, compress as many episodes onto as few discs as possible (although reviews indicate that the compression isn't too damaging as the HD files have margin for reduction).
A Different League: In contrast, Turbine Media Group is a big budget prestige operation; their website sells blu-rays of classic horror movies from the 1970s to 1990ss in HD and 4K with steelbook packaging, vintage packaging art that's had digital aging effects to create a sense of authenticity.
Rescans and Remasters: Turbine advertises new releases of films where they have done their own 4K transfers and sound remasterings of DRAGONHEART, AN AMERICAN WEREWOLF IN LONDON, TWISTER, CRASH by David Cronenberg (and with Cronenberg's supervision). Not all of their blu-rays claim to be new transfers, however. It looks like they do remasterings if the files they've received are not up to their standards. Even for the blu-rays where they seem to be using the studio's files, they have Turbine-exclusive commentaries such as with CANDYMAN, THE HILLS HAVE EYES and AMAZON WOMEN ON THE MOON.
Existing Infrastructure: This tells me that Turbine has a facility for rescanning film negatives and videotapes into high quality digital formats, remastering sound and audio, and recording audio commentaries, and they were able to put SLIDERS master tapes through their in-house processes rather than taking whatever NBCUniversal sent Mill Creek and Netflix. It would definitely have cost Turbine time, but they wouldn't have had to rework their operation or purchase new equipment because they were already using that equipment for their remasters of TWISTER and whatnot.
Season 1 Restoration
It takes 10 hours to upscale each episode of SLIDERS and that's after taking some time to copy the files to my hard drive and convert them to constant framerate with progressive video. Season 1 on the blu-ray looks like a good DVD transfer: clear but lacking in fine detail and a little fuzzy. That fuzziness is accentuated when stretching the image from 480 pixels to 720p; I think Topaz can likely reduce that. It's going to take me about 3 - 4 days.
The Pilot looked like an okay DVD video. Upscaling made it a very good DVD video. I expect the rest of Season 1 when upscaled will fall in line and look like good DVDs. That's nice, but not at the same level as the Season 2 blu-ray SD episodes which surpass the limits of DVD quality.
An Impromptu Scale
Let's create a scale of HD video quality. We'll put a blank screen at 0 out of 10. And we'll consider, say, a 1080p blu-ray of the latest Marvel movie at 10 out of 10.
Abysmal to Poor: I would put the Mill Creek DVDs at 1 out of 10 which is to say that there is a video image and it qualifies as being more than a blank screen. I would rate the Universal DVDs at 3 out of 10; they're viewable but blurry and covered in compression artifacts.
Middle of the Road: I would rank the Universal DVD version of the Pilot episode and the blu-ray version of Season 1 at 5 out of 10. Not overcompressed, decent enough DVD quality, but still on the blurry side.
Better Than Average: I would put my upscale of the Pilot and Season 2 - 5 episodes at 6 out of 10: serviceable, not distractingly poor, nowhere near a 1080p Marvel movie release, but certainly more good than bad.
Very Good: However, I would put the blu-ray SD release of Seasons 2 - 5 at 7.5 out of 10 in terms of HD presentation. They are still not HD, but the flaws of SD on an HDTV are largely absent and all the merits of sharpness and detail and clarity are strongly in evidence. Put your face up to the screen and you can see it's blurrier than a true HD image and short of the crystaline quality of CAPTAIN MARVEL or a Netflix episode of RIVERDALE -- but who actually watches TV that way?
I can get the other Season 1 episodes to 6 out of 10 with Topaz. However, it's really just for my own personal viewing. Season 1 on the SD blu-ray looks good enough and most people who'd play SLIDERS on blu-ray have a player or TV that can sharpen up the image.
Future AI Upscaling
Once Season 1 is finished, I'll certainly AI upscale "As Time Goes By" to 4K. However, it could take 24 hours and my expectations are not high.
Topaz upscaling does not actually add detail to the video. Instead, it removes flaws. When you stretch the Universal DVD version of the Pilot or "As Time Goes By" from 640x480 to 1080p on an HDTV, you get an image that is stretched to the point of blurriness and the blocky artifacts of compression are even more present.
Cleanup: Topaz removes these distortions of stretching and compression with clever combinations of increased pixel contrast that's applied specifically to different categories of texture. Topaz's result is still a stretched image, but with all the blur and artifacts alleviated with pixel contrast and deblocking so that it resembles an HD image on an HDTV. However, that high definition look is an illusion; Topaz is not adding any detail, it's just clearing away the obstructions that distract from the detail underneath the stretch and compress.
The fogginess of the S1 episodes even in an uncompressed format is, in its way, a form of compression. It's from crunching down a 35mm film image to a standard definition videotape. And Topaz can alleviate some of the fogginess of the poor quality film-to-tape transfer in S1.
Hitting the Ceiling: In the case of the S2 - S5 blu-ray files: there is no fuzziness. There are no artifacts of compression. I don't think Topaz can do any more cleanup because the image is already clean. It's a standard definition 640x480 image, but the detail and depth contained within those 640x480 pixels are sufficient to scale to an HD screen without blurriness. The SD blu-ray version of "As Time Goes By" has more film graininess than the upscale. Within that SD graininess is texture that shows you pores in the actors' skin and their individual strands of hair.
I don't believe Topaz can improve this image. Topaz tends to smooth out grain in video files when upscaling them, extrapolating and increasing the pixels around the grains to create the illusion of detail, and ultimately producing high resolution video files that lack film grain texture. Topaz would likely, in bringing these files from SD to HD, cover all this detail with a waxy sheen. I suspect that the blu-ray video files, outside of S1 needing a sharpening pass, are best left alone.
My view: S2 - S5 on blu-ray are the best possible digital versions of the standard definition master tapes. They can't be improved by our current level of AI upscaling technology; at this point, the only improvement would be to rescan the original film to 4K and rebuild HD versions of the episodes.
(It'll be good to find out for sure, of course.)
Standard Definition Detail: Also, my upscaled video files, for wider shots, often had a watercolour blurriness because the AI couldn't lock onto any fine detail in order to increase it from 480 pixels to 720p. Examples include Arturo-2 holding his book in "Prince of Wails" with the text being unreadable; the stairs to Quinn's front porch in "Into the Mystic" being a mess of overlapping lines, or the sliders standing at a distance at the end of "Time and Again World". The upscale made these shots look like garbled video data. But in the blu-ray SD version, these shots look deliberately soft focused.
The blu-ray is just better. The DVDs have been garbage; my upscales have been garbage with some polish. Get the blu-ray. :-)
I will try AI upscaling the blu-ray SD version of "As Time Goes By" to 4K. It will take at least a day. But based on what I'm seeing, the results will probably be the same as having Handbrake stretch the image to 4K in 20 - 30 minutes and it will lose image detail and fail to gain anything.