How do you feel about matrix? I've heard it as a counter point to the point you made but never really looked into it.
IM is also not really a great replacement for email.
Could you expand on this? I haven't actually noticed anything detrimental, and resource usage has actually been falling as the dev team makes improvements - though in fairness I'm running it on a beefy server that also hosts Plex, Polaris, and more.
a, fragile, depends on a lot of external libs (eg. hazmat, which needs sslv3 compiles openssl)
b, eats a lof of resources when using medium size and above federated rooms
And there are no other servers ready for production.