Having see what has happened (is happening?) to Mastodon, I can see where Moxie is coming from; as much as I hate to accept it.
Moxie also doesn't seem particularly happy with the situation; notes in the same post:
"Truly though, I wish you well in the endeavor, it's something that I'd love to be proven wrong about."
He notes two issue in particular:
- Degradation of UX
- Loss of development effort
From what I've seen, Mastodon suffered from similar problems (certainly as a user, I can attest to the first one). It seems those are inevitable consequences (along with performance/scalability issues) of the loss of control that comes from federation. Personally, I don't think those are necessarily insurmountable, but they are non-trivial, and will require effort and commitment -- including from the end users -- to resolve.
All other things being equal, a project focusing on federation will be at a disadvantage compared to a centralized one when it comes to delivering good UX. So lamentably, I don't see a federated platform becoming mainstream outside tech culture, and that is what Moxie's vision for Signal is...
[edit: grammar, formatting]