I reckon this is the usage distribution of Lemmy servers that we'll end up with.
Incredible to think about that we got it right the first time (with email) and still had to spend the last 20 years complaining about centralized social networks.
The thing about email is that the software is proprietary. Each of these providers has their own implementation of the interface, features, and integration with their tools (Google drive, photos, etc).
As long as lemmy servers run lemmy software, this won't happen, or at least won't be an issue as you can move to another server and not have to change your usage habits.
However if some server owners decide to fork Lemmy and develop their proprietary server, overhaul the UI, add features and attract users, it will start to become a problem.
email is a standard protocol. You can run your own server using FOSS software in 5 minutes if you start now. One of the biggest problems is that you will have a hard time "federating" to gmail and others due to the spam problematic, but that is something that we will see with Lemmy, too. Currently I can spin up my own server and start pushing shit to lemmy.world and other bigger instances, but I feel that this will change with the coming spam waves