I think you’re misunderstanding how copyright works. Losing the copyright doesn’t mean they won’t be able to make new works, nor does it mean those new works won’t have copyright. Copyright is only lost on the original work, so while others can use Steamboat Willie, and that very specific version of Micky Mouse, Disney still owns modern updates to him. Either way, the end of that monopoly opens more avenues for newer authors to build on it, while again, doing nothing but reducing Disney’s passive income for work their founder did a century ago. Its a more physical example, but along the same line of logic, if I cure cancer, it might make sense to give me time to get a head start on profiting from it (so I am rewarded for my work) but it would be ridiculous to say no one is else is allowed to use my cure for cancer or build on it for the next century or longer. Theres absolutely no reason not to allow the ideas to spread once the author has had plenty of time to make a profit.