I completely agree that Mastodon isn't the new place, and I am increasingly thinking Threads may not be either (at least not to Twitter level, or not for a few years), however I'm very convinced that Twitter is no longer the place either.

The technology is crumbling (see rate limiting, interaction counts), the product is moving further away from what people want not towards it (see Zuck's tweets about Threads to see what popular product direction looks like, even if it's not perfect), the ad quality has dropped very noticeably suggesting they've lost good top paying advertisers, the new ad payout program suggests that their ad inventory is low anyway (see also interaction numbers here too), and the culture has gone from pretty bad to openly hostile to large swathes of the population, at least among English speakers (see anti-trans trending topics, rise of hate speech).

I'd agree that reversion to the mean and people going back to what they know is the most likely course of action in most circumstances, but these are not most circumstances. Twitter can't recover from this, at least not without a change of leadership and many years to rebuild.