"It is _very_ frustrating that the internet has had messenger fragmentation since the beginning of time"

If you ignore IRC...

"Signal just works."

It also contributes to the fragmentation you complained about, because it is closed -- no federation, no third party clients, not even a web client. If you are on a platform the Signal team does not have time to deal with then there is no option for you. If you have a specific need that the Signal team cannot take the time to support, too bad. More problematic than lacking federation is lacking any third party client software.

Whenever I confront developers about this the conversation usually results in someone demanding a specific example of a user need they are not meeting; if I give one, they say, "Oh, we are planning that, but we first need to do $XYZ!" and if I say "there are needs we may not be aware of" they say "Well we are trying to do $this or $that, we can't solve everyone's problems!" As if users who have unusual or overlooked needs deserve to be cut off from everyone else...