At least RCS is an attempt at being a cross platform
standard, even if it still sucks.
The E2EE part, which does not exist in the RCS standard itself, is a proprietary Google thing with keys managed by Google. It is not open, and opening it is not trivial because somebody has to be an authoritative key source etc.
If not for that part I'd agree with you.
Best option is to just use a different app that just works
on all platforms. No RCS, no iMessage.
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.