Twenty years ought to be enough to recoup your investment, but I'm open to compromise. In any case, (rougly) 150 years, as it currently is, is way, way, way, way, way, way way too long.
Isn't letting them keep their copyright for long, an even stronger inventive for OTHERS to come up with some new characters? (in order to compete with them).
No, it wasn't.
The first copyright law adopted under the Constitution may have established that length, but that's a very different thing.
Ever notice how restaurants and movies never use "the Happy Birthday song"? That's because Warner Bros claimed copyright on it for decades. Another part of our culture that can't be used thanks to copyright.
There are plenty of films, plays, and TV series based on works in the public domain. Romeo and Juliet, Pride and Prejudice, and Sherlock Holmes to name just a few that come to mind. Should those creators be required to pay tribute to the descendants of the long-dead authors in order to use those parts of our shared culture?
Being granted exclusive protections during your lifetime or for a reasonable period of time after the creation of a work is defensible. De facto perpetual protections afforded to the artist's grandchildren (or great^n-grandchildren), not so much.
Could Disney have become this successful without free access to public domain works like Cinderella, Snow White, Robin Hood, The Jungle Book, One Thousand and One Nights, The Sorcerer's Apprentice and so on and so forth? Possibly. But insisting that their works should be protected far beyond the protections their source material ever had is hypocrisy at best, anti-competitive at worst.