Having worked at Twitter on abuse, I think that Twitter could do far more. They've gotten better, but it's still trivially easy to evade bans, the moderation decisions are frequently terrible, and there are a zillion other things wrong.

That said, I think an ad-funded global discussion is impossible. You can't do good user moderation with that little money (~$0.10 per user per day total revenue, with a tiny fraction of that going to abuse prevention). I think Twitter should allow people to create semi-private spaces, zones for discussion where users can be much more firm about moderation. It should also make collaborative blocking and muting much more effective.