I would argue that the user distribution does not matter. What's important is eliminating network effects, which you get with federation.
I think most people would consider email a successful federated service, and yet far more than half of users are only on the biggest three servers.
https://www.zdnet.com/article/whos-the-biggest-u-s-e-mail-se...
I don't know about Mastodon's case, but in Matrix the extremely low popularity of alternative servers is kind of a big roadblock.
This sounds like the opposite of the point I was making. What is this a roadblock to?
There are more possible stances than just "we disallow third party clients" and "we strongly encourage third party clients".
Still, I would like to see proper use of encryption by Telegram. Secret chats have no meaning if no one uses them.
Both of these will probably have a further decentralising effect.