Inflow is partially bots and sockpuppets-- and their share probably goes up the more toxic the environment is.
Have you seen the truly awesome GPT3 authored navy seal copypasta?
Perhaps twitter will eventually be autonomous machines throwing vicious and brilliant insults at each other.
Bots are humans until advertisers decided they aren't.
You've referenced engagement first, but then, you've ignored about it in your conclusion. That is, as long as the inflow counters the outflow of participants, the situation will not improve or will even worsen - just as we've experienced in the last decade.