I'd say not being able to message ~85% of smartphone users is a pretty big UX flaw.
Given this pretty graceful degradation, what do you mean by not being able to message 85% of smartphone users?
While WhatsApp/Telegram messages are actually free if you have got data.
And nowadays I get absolutely zero SMS's from people. All senders are now just bots and notification services.
I'm more or less OK with the idea of Apple not developing for other platforms. However, they are actively hostile to anyone who has offered and tried to get interop working on other platforms, which is really a shame.
Whether this limitation is intentional is irrelevant, the UX is still bad.
https://support.apple.com/en-us/HT207006
A single contact / conversation can even mix and match.