Some of them are just too far invested to see the light. It's actually pretty sad to see twitter addicts fail to migrate to any of the plethora of alternatives.
Hell no. If there is one thing I learned from moving over to fediverse, it's that people are what make the site enjoyable or frustrating. As it is now, I enjoy the userbase here and on Mastodon, and the last thing I would want is for the most insufferable of Twitter users to come and ruin it. I'm happy most people who are quitting Twitter are joining the other alternatives. I would rather people join the fediverse as a more deliberate choice based on what we offer.