These days kids barely know who Donald Duck or Mickey are. I wonder how much of that is because Disney intentionally invests in other IP that isn't under this threat.
Also, when I saw Frozen at the theater a couple of weeks ago, there was an old-style Mickey cartoon before the picture (albeit one that broke the fourth wall in ways I don't believe the old cartoons did).
Ignoring the fact this is likely false, this has nothing to do with copyright. It's a matter of trademark law, and Disney would in no way lose the trademark over any of its characters if certain specific films featuring them were released into the public domain.