How long would you say it takes to change your username and pfp on Matrix if you know exactly what you're doing and have done it plenty of times before? 20 seconds? Pluralkit can do a proxy in 2 seconds. 20 seconds is not an acceptable delay to sending a chat message to most people, but 2 seconds is.
And what if two members of a system are in a conversation, possibly with a third person? Is it going to retroactively change the username and pfp on the old messages? Cause that would turn the conversation to complete nonsense. Imagine I'm having a discussion with some friends, two of whom are in a system, I step away to go to the toilet, and when I come back 5 minutes later I can't tell who was talking in the last 5 minutes.