You can send a SMS to iPhone users, and they'll see that in iMessage.
If you want a group chat with friends, there are a bunch of apps for that. If your friends wants those cool features but refuse to use any one of existing apps besides iMessage to talk to you, then they are not your friends.
I guess the coutnerpoint is that SMS was invented in, what, the mid 90s? And MMS invented in the mid 00s (?) when things like group chats or sending a video (!) over a phone (at least > 500KB) were inconceivable. We are long overdue for a new, modern, open standard. Apple has in a way invented a pseudo-standard and a) refusing to let others in and b) refusing to also support any other open standard. I think it's reasonable to expect we have a modern standard that supports now-simple things like half decent video transfer, files, groups, locations, etc.
the counterpoint is also that, internet is evolving way faster than those standards are able to keep up to speed.
Considering the issues folks are mentioning here about moving on to RCS and then possibly being unable to receive messages, etc... Americans can just do what the rest of the world does and use any app available on the app/play stores to chat. Whatsapp, kakaotalk, line, telegram, etc...
