What does Apple do with iMessage? I don't have an iPhone and always assumed it's just glorified SMS.
The problem OP is alluding to is that some people avoid talking with people who don’t have iMessage, because SMS costs money. So there is a bit of peer pressure to use it, and a lot of people are stuck in iOS because of that.
It’s not that different from Signal/Telegram/WhatsApp, those apps are also closed and rely on network effects. But they are free and work on other OSs, while iMessage requires a relatively expensive phone.
This is doubly true if they have ever used an Apple PC to receive iMessages; in that case it will almost certainly never reach the new phone via SMS.
I don't believe there is a fix to this; sometimes you can repair it on the other party's phone by munging the contact, but most messages will just be blackholed.
Two solutions. The first requires an iPhone (to disable iMessages on your account with) and the other does not. This has been around for years now.