Many of them already do, e.g. local booksellers. They might not be price-competitive, but if Amazon provides the
linker with no incentive to link Amazon versus linking some other bookstore (say, their favorite local bookstore), some percentage of linkers will (I would guess) defect from choosing to link Amazon.
For example, I have a site with some book reviews which currently links the Amazon page for all the books, because they pay me. If Amazon terminates its affiliate program in my state, why would I continue to link Amazon, rather than someone else? I would first look into alternate affiliate programs, like AbeBooks. If I couldn't find any worth using, I'd probably change it to just link the official publisher page for each book. Or maybe to a local-bookstore aggregator like indiebound.org. Or the Wikipedia article, when one exists.