First, anything that requires a telco to be actively involved in service delivery beyond being a dumb IPv4/6 pipe is going to be terrible. Inserting those dysfunctional companies as man-in-the-middle on a global scale ensures mediocrity. RCS is a bad idea on that basis alone.
Second, iMessage is really good. It is consistently the best messaging app I've used in multiple dimensions. I use many other messaging apps on a daily basis and this gap is hard to ignore. WhatsApp is pretty strong in the greater ecosystem of messaging apps, but it isn't up to the standard of iMessage. There are other common apps used around the world are quite a bit worse. Too many messaging apps focus on trite features while underinvesting in a strong architectural core, and it shows.
Third, if you are going to have a big tech company orchestrating all of this, you can do a lot worse than Apple. Something being cross-platform or globally popular doesn't absolve it of being run by ethical organizations. Many popular messaging apps are expressly forbidden in some contexts, so they simply can't be used. I've never come across such a case for iMessage (or Signal), though I'm sure it probably exists somewhere.
There is a case for a strong cross-platform messaging app, but RCS isn't it. And whatever it is, it needs to be at least as good as iMessage or iMessage people won't use it. Average users just want something that is feature-rich, high-quality, and without a creepy company behind it. Most alternatives to iMessage struggle to clear this hurdle, so they feel like a downgrade to iMessage users. I don't think it is a tall order to produce something to the standard of iMessage but maybe I underestimate the difficulty because it doesn't seem to happen.