The point of copyright is to promote the creation of new works. It has nothing to do with the popularity of the work under copyright or after entering the public domain. Mickey Mouse was still relevant at the time of entering the public domain, ninety-five lobbied years later.
To add it's only Steamboat Willie Mickey that is public domain. The actual character as we think of him isn't. The creator has been dead for nearly 80 years. It's absolutely insane that any of those creations are still privately owned.
Yep. But if it still commands economic value for its creator and you cut that off, then it is reducing that incentive to create and cultivate in the first place.
You are assuming that incentive to create keeps increasing with more monetary reward. Incentive doesn't start out at zero without rewards (see history before copyright) and is never infinite (what good is infinite money if you never get to use it).
Brand is valuable for consumers and protecting that brand only makes sense if you continue to have ownership and control over it.
For a non infinite money glitch version of this, see “Calvin and Hobbes”. Would the world be better if those characters could be printed on random merchandise?