Re: Next, try copyright law
One could argue that the ability of publishers to make money from copyrighted works incentivises them to engage and pay writers (etc...) and thus provides incentive for those writers to, well, write.
Many (including myself) would argue that the system is unequal and in dire need of fixing, but then I occupy that (probably large) middle-ground that believes copyright law is necessary, but the current system is not optimal.
On 'the mouse', specifically, I do see a distinction between a written work and a character. A creation like Mickey Mouse is as much a trademark as intellectual property so, in some ways, wondering when Mickey Mouse will become 'public domain' is a little like wondering when I will be able to build and sell cars with the Ford badge on them, or PCs with the HP logo.
Really, Mickey is a trademark - the character, and indeed other similar cartoon characters, are used as identifiers for their brand. The cartoons themselves are protected via regular copyright (which may or may not be extended) but the characters themselves would seem to be valid for protection potentially in perpetuity. (Apologies for the alliteration.)
Seeing an item or work with Mickey Mouse instantly signifies that that product/work is the property of or licensed by Disney. If the protections on the character and 'likeness' of Mickey Mouse were to suddenly expire, think of all the knock-off goods that would appear. It would most definitely cause 'brand dilution', which is precisely the thing that trademarks are designed to protect against.
To be clear, a cartoon with Mickey Mouse should not receive any special or extended protection (which is what you were likely talking about), any more than a song or a piece of writing should. After X years, that work should be considered in the public domain; the music can be used in a movie or a YouTube video, the book re-printed by another publisher, or a cartoon/movie shown by a broadcaster - all without having to pay anyone. BUT, that is not the same thing as someone making a new movie with the Mickey Mouse character.
Like the bunny, the mouse fulfils the criteria for a trademark and should be protected as such.
THAT SAID, the copyright laws of works do seem to get extended and I don't agree with that at all. There is no reason why the specific work of 'Steamboat Willie' should not be part of the public domain. While it might not be quite the same, modern symphony orchestras and opera companies simply could not exist without the timely expiration of copyrighted music - it is very much in the public interest that the Brandenburg Concertos or Mahler's ninth are in the public domain.
An interesting thing there, is that up until relatively recently, the US did not recognise foreign copyright laws so Disney was able to use Stravinsky's 'Rite of Spring' in Fantasia without paying the then still-living Stravinsky any royalties for the film's US release. They did have to license for international release but were later taken to court for breach of contract when the film was released on video and largely lost, having to pay up.