Now, I know you might think me too impatient and bitter and stuff, and maybe I am (waiting so long to see Voldemort defeated, as well as Chrysalis and Thanos who might NOT even die, will do that to you), but I will start by saying, for a show that's going on so long, Supernatural did good by not losing me a LOT earlier than Season 14. Pokemon mostly lost me and many others at Season 5 (though in some cases this was coz Misty left, and I didn't particularly like her at the time unlike now, though I always loved Charizard).
The Simpsons, Family Guy, Spongebob, even MLP is getting less fun for me with Seasons 7 and 8 (though many liked Season 7) and I might just ignore Season 9 (only reason I didn't ignore Season 8 was two characters I really liked that were introduced that season, but four of the episodes were poorly written and I do ignore them).
Burn Notice, The Mentalist, Buffy, and Once Upon A Time ended at Season 7. Charmed, Voltron, Game of Thrones, and Inuyasha at Season 8. 24 and MLP at Season 9 (though in 24's case it's Season 6 to me, 7 and 8 weren't fun for me AT ALL). Dragonball Z at 276 episodes (though they are making a rather long sequel series to its reboot). A show with a finite number of seasons can be easy to rewatch if you feel like it, but harder if it's too long. Game of Thrones has shorter seasons, it and shows like Gravity Falls or The Shannara Chronicles won't be too hard or time-consuming to rewatch for entertainment. (I never read the Shannara books so I can enjoy the show, please don't judge)
The problem with a show going on for too long is a number of things:
1.) It gets too hard to follow. Too hard to rewatch if you feel like it, to keep track of a specific episode. Too hard for a newcomer to get into it at this point coz they can never catch up. Too hard to fit on your shelf on DVD, and you can't rely on Netflix to watch it whenever coz they might take it off one day, which they did with Supernatural so I had to lend Season 12 to my brother so he could watch it, he's way behind and I have to keep hiding Crowley's death from him. (But he thought Seasons 6, 7, and 11 were boring)
2.) It starts to get stale after a while. Or get inconsistent. (Like, Dean said the last thing he remembered was saying yes to Michael, but he was in control when he killed Lucifer, THEN Michael took over). Again, most shows going on that long would have been "Spongebobbed" earlier, Supernatural remained interesting for me as far as Season 13, which is pretty good actually. All good shows should end on a good note. (Look what happened to the Tim Burton Batman series when Batman & Robin ruined it)
Ever since Season 1 it's been cliffhangers at the end of each season. Sam, Dean, and John get hit by a demon with a truck. Dean makes a deal to go to hell in a year. Dean dies and goes to hell. Lucifer is released. Sam is out of the cage leaving it unknown if it's him or Lucifer (though that was the least cliffhanger-y). Castiel is overcome with power and declares himself God. Dean and Cass go to Purgatory. The angels fall. Dean becomes a demon. The Darkness is released. Toni shoots Sam (the show could've ended without the lead-in to the British Men of Letters). Jack is born, leaving it unknown which side he'll be on at the moment. Michael possesses Dean.
I put up with that and - once I started watching them on TV instead of waiting for DVD's - the Christmas hiatuses that are obviously just meant to annoy us. As well as killing almost all my favorite characters. Now that Ketch reformed, how long before he is killed off for real? (Metatron's death was an episode after his redemption) Or Alternate Bobby and Charlie? Anyway, that was minor, but it just doesn't stop, the story just won't end, and that makes it feel incomplete, and watching through it all even if I ignore Season 14 and above would take months, maybe a year even.
It IS fine for a show to keep going on if each season or film tells its own independent story, like James Bond (starting from Live and Let Die), Buffy, or (for the most part) 24, where the next threat is the beginning of the next season rather than at the end of the current season. That way if people don't live to see the next season they can rest in peace, or if they feel it's getting stale they can leave whenever they want and not feel it incomplete by only watching or acknowledging up to a certain point.
I thought Episode 300 would be the series finale, but a 14th episode of the season was confirmed, and likely this won't be last season. The show will keep going until Jared and Jensen either get tired and quit or die of old age, and frankly I don't have the patience for it anymore, though it is worth noting this didn't happen until the show started to get stale to me. (I don't think Lucifer's possible revival is a huge factor in this but if Cosmic Entity does revive him as is implied, it really cheapens that spectacular flamy death, even if the fight was considered anticlimactic)
Castiel and Crowley were the only two fan-favorites they wouldn't permanently kill. But they did kill Crowley, why? Probably coz Mark Sheppard got tired of Supernatural and wanted to do something else. Also Castiel made a deal with Cosmic Entity and will likely die permanently this season, suggesting that Misha got tired of it as well. How long until Jared and Jensen get tired of playing Sam and Dean?
Thank you to anyone who took the time to read this, and I know I'm real impatient and negative, but again, this didn't become a HUGE issue until the show started to get stale. I will probably ignore Season 14 and just read the detailed summaries or character articles to update the List of Deaths Wiki, which honestly was the only reason I even watched the last few episodes. And, one more time, Supernatural exceeded expectations to keep me interested for 13 seasons.