I could not talk to all my friends on only one app, as the vast majority of people were not online. If anything, a far larger number of people are still on open services today than were back then; there are just even more people on locked down services.

But why you brought this up in a discussion about the invention of the web, I have no idea.

Do you remember the AIM/MSN wars, where AOL was literally building buffer overflows into their protocols to prevent MSN messenger from interoperating?

There never was a golden age of interoperability on the internet, at least not after HTTP. Maybe there was a lot of cooperation when it was purely academic in the late 70s and 80s. BBS operators often were cooperative. But commercial internet has always and forever been cutthroat.