My point is that it doesn't need to do that. Copyright could still encourage creation of new works with a much shorter duration, without the downside of locking our culture up in monopolies for life + 70.
You could equally solve the issue by providing provisions for archival. Or a provision that requires a work to be freely available for sale.
The creator having a monopoly on the work seems to me to encourage the creation and sharing of work as intended.
I wouldn't say we have more "traditional" art that an author spends time creating with the vision of selling copies for years to come, than we did when copyright terms were shorter.
Is there a lot of fan art, yes, there's also massive amounts of original works as well. There is no lack of either.