Warner Bros. Pictures

Actually, it is the Warner Bros. who is most to blame for destroying the movie.
If the information on Wikipedia is correst;
"After serving as animation director on the successful Who Framed Roger Rabbit, Williams got funding and a distribution deal with Warner Bros. Pictures. The film was not finished by its 1991 deadline, and Disney's similarly-themed Aladdin beat it to theatres the following year. Warner took the project from Williams, and had producer Fred Calvert complete the film. Calvert cut a significant amount of Williams' finished footage from the film, added songs, recorded voices for previously mute characters, and added Jonathan Winters' narration.

Originally Richard Williams kept all the materials for the film in a vault, which he called a "reason for living" (in the 1989 Channel 4 documentary I Drew Roger Rabbit), and they were kept very safe, but when the film was taken away from him, much of this material was sent to Korean artists who did not save any of their work - plenty of classic animation was junked."

How is it possible to destroy something a man have used many years of his life to complete just like that? There is a lot of famous movies today that would never have been made because of a budget that became too big and/or took longer time than expected, if all studios whould be just as impatient.

In its own way, this would have been a movie from the 60's.

Some more info:

"Unfortunately, Williams took such a long time to make the film that Warner's began to panic. (In fact the project had such a lengthy history, over its twenty-seven years from its inception to its release (a world record), that several of the people doing voice-work - Vincent Price, as well as both Anthony Quayle and Donald Pleasence who did voices at one point but whose work didn't end up on the finished print - ended up dying several years before the film ever saw the lights of theatres). Fearing that Thief's thunder would be stolen by Disney's Aladdin (1992), Warner Brothers panicked and called in the completion guarantors. With only 15 minutes of footage left to complete, Williams was fired and director Fred Calvert and a Korean animation-for-hire studio were assigned to turn the existing footage into something salvageable."