Where are you getting that 50% estimate from?
Arathorn mentions this in a bunch of his Matrix talks (and has mentioned it on HN too I think).
For the record: about 2.5M of these 5.6M users on matrix.org are native to Matrix, rather than bridged. the 50% guess comes from the fact we see 8M in the phonehome stats right now (including the 5.6M on Matrix.org) so that gives 2.5M on and 2.4M off. In practice the number off would be much larger given lots of bigger deployments we know about don’t phone home.
That's just blatantly false. Approximately 50% of Matrix users are on other homeservers.
> It's not so much a technical question as it is the attitude of "hey we're implementing our own chat protocol cause XML sucks".
This is just a strawman. Every single talk by Arathorn explains, in great detail, why Matrix is not just "XMPP but JSON". Maybe you disagree with their reasons, but then you should argue against their reasons not some other reasons that you came up with.
> That said, if the matrix protocol can actually manage to attract users and multiple implementations some years down the road
Given the recent hack it looks like Matrix has about ~10 million users federating with each other (if 50% of them are on Matrix.org and Matrix.org has 5 million users) -- and this doesn't count bridged users which aren't using Matrix but are benefiting from the ecosystem.
And there are also several implementations. Riot is the most popular and polished one, but there's a whole bunch of others[1].
[1]: https://matrix.org/docs/projects/clients-matrix