It kiiiind of is a zero sum game, since if you’re signed up for both, you’ll naturally use the one that gives you better results for your chosen metric. For twitter it’s usually engagement, so if you get a lot of engagement on twitter and little on the fediverse, you’ll end up focusing on twitter.
Assuming that Twitter doesn't crash and burn in the next week or so, and assuming Mastodon doesn't draw enough users to create its own network effect, sure. But then, people don't always act according to a single metric. Mastodon doesn't have ads and it doesn't follow a single moderation policy. I can see people sticking with both for different reasons.
