I just think it will organically happen.
People will happen to join one server, they get their friends to join that server, and before you know it, it’s naturally become the most popular one. Because people have signed up to that one, they also start making their own communities on there.
Then it shows up as the first result when people google “Lemmy”, and carries on growing larger than the others.
The best way I see of avoiding that, is having servers that focus on a topic I.e. sports, tech, memes, porn, knitting etc.
That would prompt people to use different servers, but also keep some logic and organisation.
I don’t think people will share usernames, but they’ll say “go here to use Lemmy”