I would frame it as "messaging applications should interoperate".
People understand that a T-Mobile customer can send a text to an AT&T customer. They understand that a Samsung owner can call an iPhone owner.
Why do they accept that a Slack user can't send a message to a Discord user? Why do they accept that Google has made 11+ different messaging applications that aren't compatible with each other? Why do they accept that you can't call a Skype for business customer with Skype?
We've had 30 years of stagnation because messaging applications don't interoperate. That's why the story is always "Hey, remember back when ICQ used to work? You should try Skype, it really works..." Once Skype becomes successful at the two-sided market game the honeymoon is over and the vendor has no incentive to keep it working and they figure you'll keep trying to use it anyway.
If there were competition for both the server and the client there would be continuous pressure to keep the products working, and to improve them. So instead of Facebook Messenger being the same as AOL Instant Messenger except for Facebook, we'd have had 30 years of progress and would have apps that look like something out of The Jetsons.