I feel the same with the "find me on mastodon" twitter bios: people will realize that only 50 of their thousands of followers can be found on mastodon, and then move back in a few weeks after giving up on the experiment. During all that time they have still kept most of their activities on twitter because there is simply more going on there, more replies to their tweets, more tweets to reply to, etc.
banning them might have actually the opposite effect here: then they get forced to focus on mastodon. Having a large number of users advertise mastodon ids and talk all day long about how horrible twitter is on twitter actually drives engagement. Banning large numbers of users is very bad for engagement on the other hand, but very good for mastodon engagement numbers.