The truth is that a profit-maximising business will seek revenue opportunities where it can, and if that means selling both services, on a single-instance or subscription basis, and advertising, it will do both.
Advertising-only or advertising-dominated businesses have a strong tendency to degrade faster and far more prolifically than those with mixed-model funding (I still find The Economist's three-legged revenues stool fascinating: subscriptions, advertising, and Economist Intelligence Unit bespoke consulting and research services, each roughly 1/3 of total revenues).
Paying alone, however, is a far-from-sufficient condition.