I typed my old Reddit username in at first then said “wait a minute…”, somehow nobody took my simple first name in the first several hundred thousand accounts created!
Usernames only have to be unique on each instance, kind of like email. Even if Lemmy is growing exponentially, not all that growth will be on lemmy.world.
I've always been able to grab my username. Of course, I originally (in a different context) tried "timholt", which was taken, then "thetimholt", which was also taken.