Why is iMessage even an issue? All phones can send conventional texts to each other
iMessage sends high quality pictures and videos, try to send a video thru mms and it is unusable. iMessage also has read receipts, you can see if they are typing. iMessage works over wifi without a cellular connection (which was a big deal, but I believe carries now support this for android phones) Also iMessage automatically just works by default. You don't download an app and make an account. So for anybody who is…
To add to what you're saying: RCS (which everyone except Apple uses) offers all of those things as well, and offers them in a way that is interoperable with all non-Apple phones.
iMessage doesn't actually do anything that RCS doesn't do. Apple simply refuses to adopt RCS (or make iMessage interoperable) because they want to use the lock-in to drive more sales of Apple products.