3 is a different number from 1. If a single instance had over 50% of signups it would be reasonable to describe it as dominated by a single big player. If the biggest instance only has 20% or whatever the reality is, then it is not dominated by a single big player.
Definitely there’s a tendency to centralize up from thousands of little shards to a few big professional units - though as we see in every one of these examples, that doesn’t mean the little ones have to disappear. You still have plenty of small email clients and small instances. What’s important is that if one big one goes down or goes evil the other big ones are there, and that there’s always the possibility of new small ones blowing up if they do something better than the big boys.