The golden age of chat was back when you could use one client to talk to all your friends despite them being on ICQ, AIM, MSN, IRC or half a dozen smaller networks.

Trillian! It was nice to use a single client for ICQ, MSN and others.

Universal availability and plain one-to-one & group chat removes most of the issues. Less important to support things like file transfer, voice chat, etc.

Who could combine Skype, WhatsApp, FaceTime, Hangouts, Discord, Microsoft Teams/Skype for Business, etc. and do it in such a fashion I can trust it.

Damn, I don't like government regulation, and when it comes to chat I'm frankly scared — I don't think they could resist the urge of adding backdoors in the process. But who else could put some pressure on the vendors to get these things to interoperate?

Adium for the Mac folks! The website is still there (https://adium.im/) but the version history page is an error and the product blurb talks about AIM support, so I'm guessing it's dead.

Chat will never be interoperable because then vendors wouldn't be able to introduce their own exclusive emojis or GIFs or stories or whatever other nonsense.

Chat will never be interoperable as long as it's controlled by for-profit companies.

I think this is the craziest part. Got fed up with your client? Well, you should wait until all your important contacts are fed up with it as well.

I can't believe I have to use a client as bad as Skype.