I kind of think you are missing the point OP, the ending doesn't retrospectively make a show bad, it removes your desire to watch it again, at which point the ending has ruined your ability to enjoy it again.
Take Game of Thrones as the example, which wasn't just the ending, but the entire last series (we'll come back to Dexter or TROS).
GOT it was a fantastic series, it was so detailed, its world was fantastic, you could enjoy it at face value, you could enjoy the little hints and the theory crafting but however you enjoyed it, it was building, building to a huge pay off. But then it didn't, the directors/writers D&D without source material couldn't write for shit, they floundered and it showed.
Perhaps the bare bones plot is the end game for the series, for GRRM, but their decision to speed run the series to move on to other projects, or because they didn't have the ability to flesh it out to full series (and HBO wanted more and offered more money), was so painfully transparent it utterly shat on the previous series, scope, lore, ambition etc. It didn't retrospectively make the entire story bad, but it killed the ability to rewatch it, why invest in 7 series (some will argue 5 or 6) if you know the quality dropped to a point that made little to no sense?
Story lines were telegraphed through series after series only for in the final series to be abandoned or watered down (Jon being a Targ, Arya's face magic, Jamie's redemption etc), i'm sure with a full series 8 and 9 you could give those story lines relevance, you could flesh them out, to make the end seem appropriate, but they didn't. The cast hated and didn't understand it, so did fans.
The journey was brilliant, the pay off was awful, so why reexperience the great journey knowing it would be a poor pay off?
GOT is probably the worst example of this as we've gone through a pandemic and nobody is rewqatching this, there are planned prequels in the universe and there is no hype or desire for them, with Star Wars thats not he same.......
Similar things occurred to the Sequel Trilogy, but its problems were more due to poor planning and execution. unlike GOT where they had source material* to follow, instead of creating an overarching plot for a planned trilogy, they treated every film as its own thing and let writers/directors have carte blanche, with no direction. This created 3 movies that when viewed in isolation were good to various degrees, but as a cohesive trilogy? no they failed, TROS was the straw that broke the camels back! TLJ wasn't a terrible film, it had some questionable storylines and lore, that didn't really allow for 1 more film to tie up all the plots. Arguably TLJ would have been better as the first in a trilogy and they could have followed on, but they didn't and TROS almost became a fan fiction rewrite. TROS was dumb fun but as an end to a trilogy all it did was highlight that the trilogy wasn't just ill-conceived, it wasn't at all.
Its the closest one to retrospectively making the series bad, in that it created a situation where when viewed as a trilogy they simply do not work cohesively, but in reality in isolation or with a lot of suspension of disbelief they are still fun.
Now people bring up Dexter and that is similar but also different, it didn't have the same over arching plot that the above suffered from and you can quite easily watch the good again and stop, there is no continuing plot you need to keep going for.
TL:DR they don't retrospectively make something bad as such, they retrospectively remove the desire to rewatch as it means however good the journey was, you are traveling to a garbage dump and that what is occurring.
