That is a concern. But I suspect the market for a moderated ad-supported Twitter is quite a bit larger than that for a paid unmoderated Twitter, and the elastic

That is a concern. But I suspect the market for a moderated ad-supported Twitter is quite a bit larger than that for a paid unmoderated Twitter, and the elasticity you have to "promote hate speech" without really alienating people is not large — people already got mad at Facebook for counting "angry react" as engagement, and that's not even close to being hate speech. On the other hand, it also incentivizes aggressive moderation of the free section, since malingerers can be told to pay up.