There's no real reason to believe that, though. If someone has a space for an ad, why wouldn't they sell it, even if they don't need it to produce the content? This is one of the problems with profit-maximization: it means every avenue of efficient revenue generation should be exploited whether it's needed or welcomed or not.

Even the pay-for-no ads model doesn't hold up, because if you pay for content, why wouldn't they just double-collect and make you pay for ads served with the content? I purchased my phone and my phone service, but I still get ads in my notifications. Because I didn't pay "enough" to avoid it.

It's like paying off a blackmail ransom. You give them $100 and they come back next week and say "how about another $100?"