Except you're very wrong. A quarter of a million people registered Mastodon accounts in the last 24, thousands per day prior to that... people are joining established communities and creating their own. And it's actually a lot nicer, in my experience (and I've heard the same from everyone else.)

Creating an account on another service doesn't mean you've left or are leaving the first one.

How many people deleted their own Myspace account? If critical mass is hit, critical mass is hit and there's no turning back.

Still it's a lot different from yelling "I'm leaving!" and then do nothing. They now are exposed to the "competition", which is dangerous from Twitter's point of view, because now all it needs to lose those users is a bigger enough users base that attracts them into Mastodon, and the more users migrate, the faster the migration becomes. In electronics that would be called an avalanche effect.