The value you are talking about is all because of SMS. It's the lowest common denominator and IMO shouldn't be combined into a single app.
And there's definitely no reason why either iMessage or RCS E2EE need to be locked to a specific platform. Signal, WhatsApp, etc just work everywhere with no quirks when messaging people on different platforms.
If not for that part I'd agree with you.
Well, I think there's obviously a huge place for these apps and there always has been. There is certainly nothing stopping you and your buddies from all standardizing on Signal, Telegram, or uh.... buying the rights to ICQ and resurrecting that or whatever.The value of iMessage/RCS/SMS is that it is effectively universal. I just need somebody's mobile number and I can call or text them. They are (more or less) guaranteed to be able to receive that call or text. I can buy the most advanced iPhone or Pixel and I can send a text message to some dude on a 2001 flip phone in a jungle somewhere. That is a huge huge huge value.