That's entirely irrelevant though. The point of copyright isn't to protect income. The point is to encourage the creation of new works. Disney doesn't need 100+ years of exclusive profits on something to encourage them to create new works. Nobody does.
I'd even argue that the more popular a work is the more important it is that it enter the public domain sooner rather than later. The less cultural relevancy something has when it enters the public domain the less likely it will inspire new works to be created.
Suppose Copyright as a concept was overturned and no longer existed. Would Disney just say "Well, it was a great run, but we're going to close up shop and no longer create works." Would an independent artist who needs to paint something decide not to just because it couldn't be copyright?
"The creation of new works" doesn't need to be encouraged. It's the default. Cavemen still carved on cave walls without copyright.
Many works require a good deal of investment and time and if people had little to no chance of making money or breaking even on that investment a lot of works wouldn't get made.
Another nice aspect of copyright law is that it establishes where a work originated. Authorship gets lost in a lot of the things we treat as if they don't have copyrights. For example memes, or the way every MP3 of a parody song on P2P platforms ended up listing Weird Al as the artist regardless of his involvement. It also happens in cases where copyright really doesn't exist like with recipes and as a result we don't really know who first came up with many of the foods we love. A very limited copyright term would more firmly establish who we should thank for the things we enjoy.
That's, by design, the tool used to encourage people to invest their time into producing works.
We would not be having this conversation at all if people weren't able to make money of these works - there'd be no point to copyright at all if there wasn't money to be made (by the artists) and the reproduction of their works wasn't restricting their ability to generate that income (for themselves, or their agents).
I want to emphasise that I am not arguing in favour of the system, only how and why it works this way.