AIM, Discord etc. have the 'word-processor problem': the protocol and the interface are inexorably intertwined. You're stuck with whatever user interface they provide.

And this 'word-processor problem' is compounded by the 'normal people' issue, or, even worse than that, the 'lowest common user' problem. It's optimisation not for the best or the most powerful or even the prettiest interface, but rather optimisation for ease-of-immediate-use. (I.e. the interface for word-processors, whether LibreOffice or Word, are miserable environments for actually creating in, because they're tailored to not even 'average users' but to 'inexperienced and lazy users' - those who want to be able to sit down and use it without any training.)

With an open standard, you can have good interfaces for 'normal people' and an interface for 'lowest common users' and interfaces for other types of users, and this enables all of these types to be able to interact.