Don't you have that backwards? If I wish to make cartoons and use an anthropomorphic mouse, I have more incentive to create a new character so that I can avoid having to license Mickey. If Mickey were public domain, I'd have less incentive to create a new character, as I could just use Mickey.
In fact, I might have incentive to specifically avoid creating a new character, because Mickey has recognition.
What if Disney finds the character resembles too much a copyrighted character?
If we plan on extending copyright forever, we will end up with a pretty crowded characterspace in a couple decades. Are you sure you'll be able to come up with something that's completely original a couple decades from now? Is it sane?
In fact, I might have incentive to specifically avoid creating a new character, because Mickey has recognition.