The Internet Archive model of CDL to everyone on the Web, not just local library card members, is a particular threat.

There are many books for which a single CDL copy is probably sufficient to satisfy the entire global demand for that book. They will be referenced from time to time by academics or independent researchers and almost never read cover-to-cover.

If every college/university and big city library needs to own a copy so that their members can get access to it, that's not such a big problem, you can still sell maybe 1,000 copies. But if everyone can share a single copy, that is a big problem.