Yes, but if you can accurately predict the CPM ahead of time (without hurting conversion rates for high-CPM users), you could deny service to those users whose CPM wouldn't cover costs or required profit margins.

Of course, in reality the marginal cost of serving any given user is near-zero so ad-supported services are happy to serve just about anyone. That's why they'd survive just as well in an ad-less world as long as some minority of users is willing to pay.