If people valued the existing sites enough to pay for them they'd already be doing that, it would simply be a more reliable stream of income.
I do agree that most people won't prefer paying with money instead of attention, but that doesn't mean they won't prefer paying with money over not having the service.
This dynamic is even more powerful for network-effects-driven platforms like Twitter or fb
When people perceive a value, they'll pay an appropriate price in some way (be it by spending money or watching ads), that's all I'm saying. And they appear to value social media or they wouldn't spend that much time on it. So if you remove "just watch some ads" as a payment form, I'm pretty sure that at least some will switch to handing over money.