I think it could work. The set of users who would pay is a completely different demographic than those who are following Justin Bieber. You're only devaluing the market for the adverts that would appeal to everyone.

Aren't "users who will pay" the most desirable demographic for marketing to?

Different groups of people will pay for different things.

Beiber followers might not spend money on Twitter, but they might on whatever tat it is teenage girls get their parents to buy for them.

The kind of people who will pay to opt-out of adverts are probably not the kind of people who will click on your adverts.

Users who will pay for twitter API access to use as infrastructure != users who will pay in general

Understood, but I'm wondering if there's a compromise somewhere.