I'm not sure what the argument is here.
That because there's a large corpus of public domain works, then the long copyright protection is ok? That people want a short copyright protection because they're done with everything in the public domain?
Would that also imply that if the number of public domain works gets large enough, then the duration copyright protections should also increase?
I think it's selective outrage and people really don't care what happens to Micky Mouse.
I'm not a fan of Disney, but I don't think my life would be better if we saw a bunch of clones from China because it's now "public domain".
Long copyright protection is not okay, but letting the huge corpus of existing public domain works languish in obscurity is not okay either; that does a lot more damage to our shared culture, and in a way that's even quite easy to address. But the damage done by keeping works in copyright is easier to see than the damage done by not making remarkably similar works accessible at all.