Do you think the ending of the Fox and the Hound was Disney's greatest letdown EVER?
Not at all. Even before I learned my preference of sadder movies, music, etc. I never had a problem with the ending and wish more animated films would end like Fox and the Hound, Alpha & Omega, The Land Before Time, etc.
Emotional or sad endings, but in a good way. All three of the movies I mentioned could have taken different routes that are sad in a more negative away.
Fox and the Hound: Tod and Cooper dying to a bullet like they did in the novel.
The Land Before Time: There were plans for a different ending that implied the GOF had died and were in heaven when they arrived at what we thought was the greay valley, if I remember right. (Correct me if I'm wrong.)
Alpha & Omega*: There weren't any plans for this, but imagine Kate dying instead of having a near-death experience similar to Tod in Fox and the Hound. (Don't forget, A&O and Fox & the Hound have the same writers, producers, animators, etc. as the creators of A&O left Disney over disagreements with how the films should be made and started their own studio continuing the old Disney style mixed with CGI rather than what Pixar and Disney do now.)
However, there are two deaths in Alpha and Omega 4: The Legend of the Saw Tooth Cave, as well as it taking a much darker turn than the first three films ever did that makes me like it better than any other animated movie I've seen [edit] not since 2010 but in my life, actually [edit], even the first Alpha & Omega movie itself and The Land Before Time.
Now don't get me wrong, I've got nothing against killing off main characters (my T- and M-rated Alpha & Omega fanfics show that quite often with the lead characters from each of the movies) but I don't hate the ending of Fox and the Hound either.
*Assuming anyone reading this has seen Alpha & Omega, that is. It's still one of my favorite animated movies, as are the second and fourth films, and it ends similar to Fox & the Hound in terms of a mix of an emotional and good ending.
(I don't think I've ever written this long of a post in the five years I've had an account here, haha.)