In leaving his show, Jerry O'Connell was ridiculous, refusing to perform an exit story for SLIDERS. In leaving her shows, Katherine Heigl was ridiculous, whining about how she hated being on ROSWELL and GREY'S ANATOMY for the long hours when she knew what they'd be when she took those jobs. Wil Wheaton was also ridiculous, quitting STAR TREK after three seasons because he believed regularly being in people's homes every week was somehow holding him back from superstardom.
But Jared and Jensen -- they've been the leads of a show that has them in nearly every scene for 15 years. When SUPERNATURAL first started, their characters would pretend to be community college students; now they pretend to be FBI veterans. The recent 20 episode orders are to give them a longer rest after over a decade of 18 hour days. They've profited greatly from 15 seasons of pay, royalties, merchandising, conventions and other businesses. In return, they've done their part and more for the show and the fans.
The real disappointment, for me, is that WAYWARD SISTERS wasn't picked up. Had the spinoff been successful, the SUPERNATURAL universe could have continued in that form to cushion the blow. It is likely that SUPERNATURAL will continue after its conclusion as a digital comic that Slider_Quinn21 won't read just as SMALLVILLE and REVOLUTION did for a time and there could be a WAYWARD SISTERS digital comic as well that Slider_Quinn21 won't read.
SUPERNATURAL doesn't have anything left to prove or achieve at this point, so the reason why its departure is painful is because it had become an institution. I can't remember my life before watching the show; I am not sure if I even existed before its premiere and that's insane. I watched the pilot and then didn't watch it again for about eight years, getting caught up only because my niece was obsessed with it and I had to watch it to understand anything she was saying. Seasons 1 - 2 were a poor X-FILES clone, Season 3 found its feet and I've enjoyed every season of the show and outside of killing Kevin and Charlie and the alternate universe hunters, I've never felt hostile to the series or felt bored with its content.
FRINGE is often considered to be THE X-FILES done properly: it featured FBI agents investigating the paranormal, it played out its five season arc, it had running plotlines that were sustained and concluded, it had great love for its characters and gave them continuing and climactic arcs. But I think SUPERNATURAL is the true successor to THE X-FILES. Yes, it chose the supernatural over the FRINGE choosing the technological. It also features Chris Carter's multi-genre anthology attitude but, unlike Carter, the SUPERNATURAL writers were careful to keep Sam and Dean's characterization consistent even if they'd been in a splatterfest last week and were in a metatextual parody this week. It features lengthy arcs like THE X-FILES, but sustains the arcs even through the standalones. It ensures that monsters-of-the-week are thematically tied to the arc even if they aren't situationally connected.
Almost everything THE X-FILES attempted, SUPERNATURAL perfected aside from its portrayal of women. THE X-FILES inspired a generation of women to go into science, engineering and medicine; SUPERNATURAL wanted to inspire women to go into law enforcement and the military but WAYWARD SISTERS didn't make the CW's cut. Both SUPERNATURAL and THE X-FILES were renewed well beyond their intended or natural lifespan and SUPERNATURAL wrapped up its original myth-arc and conceived new ones while THE X-FILES stalled. THE X-FILES had a revival and still left us on a cliffhanger. SUPERNATURAL will end.
I kind of hope that there might be a revival (not a reboot) every 3 - 5 years with Sam and Dean in a six episode mini-series whenever the actors are available and willing. SUPERNATURAL conventions will likely continue for at least another ten years as 15 seasons gives actors lots of amusing on-set anecdotes.