Pretty sure that most of new users will not make the switch and instead keep using Twitter, or another big social network in place. There have been other waves of new users in the past. If people keep joining the "main" instance, it will not promote federation, which is the whole point of using Mastodon in my opinion.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!