We have that today though - it's called Matrix. While other platforms aren't literally built upon it like Hangouts was in the beforetimes, it allows inter-op with more platforms. Whatsapp, Telegram, Discord, Facebook Messenger, Signal, Google Chat (née Google Hangouts)... and those are just the ones I'm using. The full list includes more [0]
We can disagree about what counts as good enough for the mass market in the modern era, whether normal people will actually use this vs whether normal people used XMPP's interop in 2005, and quibble about feature sets (video calls weren't initially supported in '05, and most bridges don't support them today), but for chatting with friends, you still only need one app - and because Matrix is an open standard, you can even change which app you want it to be.
Post script: I look forward to hearing about how terrible Matrix was, last time someone tried the (over-crowded) default server 12-18 months ago. The software dev community here will follow up and say that software cannot have gotten better since then, either.