If setting up and maintaining both as supported and fee-based infrastructure had no costs to Facebook and didn't hurt their advertising revenue per ad-supported user, sure, that would be the case.

In the real world, first, setting up and maintaining such a system would have substantial initial and operating costs (and possibly engagement costs, as it would complicate the UX), and, as opt-outs feom advertising would tend to be weighted toward the wealthier, more valuable (to advertisers), users, it would drop per advertising revenue per remaining advertising-supported user. So the buyout cost would have to be substantially more to break even.

I doubt it. You're paying cable television and still see ads. They'll always be companies willing to give Twitter money to be able to advertise to them.

But how much would cable cost if the individual channels didn't rely on advertising for profit?

They'll never do this.. it is way more profitable to keep your value opaque. The average user might be worth $13 but not you!