This is what the internet used to be like. Nowadays, something like this would be unthinkable. But you could use Pidgin to connect to Facebook Messenger, AIM, ICQ and MSN. No, you couldn't necessarily use your AIM account to talk to somebody on FB, but that was only because the companies making these services didn't implement this. The fact is, it could've been done if they wanted. The technology was there, the environment was there, the open standard that almost everybody was using was there.
Now you have Discord, Facebook, etc. banning third party clients and enforcing a walled garden when they don't have to. It's a conscious choice they're making.
I'm not sure why you're defending the current state of things. It's worse now.
