As far as I can tell Mill Creek isn't in the business of colour grading. If I had to bet money on it I'd say they are probably just doing the disc authoring & printing/pressing. I could be wrong.
I wouldn't even mind the lower 4.5mbps bit rate of the Mill Creek release if they had at least used the same masters as the Universal version. As an example I just bought this knowing full well that one of the reviewers has mentioned the reduced bit rate due to the lower disc count compared to the original 2008 CBS release. Although if it has serious mastering errors it's going back on ebay.
You'd think over time the image quality would get better, but with DVD they are actively making it worse. Which really hurts since DVD is already suffering at 720x480 standard definition, and there is often no better option due to the streaming services being even worse (except of course for shows that were remastered in HD at great expense to the studio).
Given the surprising popularity of DVD some 28 years after its conception, maybe they should consider coming out with a "DVD+" spec for better quality video while retaining backwards compatibility and cheaper production costs compared to bluray. For example the DVD+ spec could specify a minimum bitrate and have predefined encoder profiles agreed upon by industry experts. They could retire the interlaced format and require all video be stored on disc as progressive, and if generated from an interlaced master it would need to comply with a certain standard of deinterlacing & ivtc defined by industry experts. The raster size should also be expanded to 852x480 for NTSC and 1024x576 for PAL to retain 4:3 pixel density at 16:9.
Yea, you are right, they don't do color grading as far as I know. They did have Quantum Leap on Blu-Ray HD, and Airwolf, but I think maybe Universal did the restoration on that. Mill Creek I think usually just uses the files universal gives them (and actually contractually these types of deals with distributors don't allow them to "change" the content).
As far as bitrate -- could that just be a product of them trying to fit more episodes per disc? Because even an added disc cuts into Mill Creek's slim profit margin.
That, or it's just them being lazy or not having the know how.
You're right about DVD though -- I am constantly surprised how it's still ~40% units sold for the home market. A lot of people stuck on dvd for some reason.