An app falling back to something else entirely does not make the first system "compatible". All Apple did was bundle SMS into their iMessage app. That has nothing to with iMessage, which is not allowed on non-Apple hardware.
Google Messages and every other default messaging app on phones falls back to SMS when RCS isn't available for the recipient. Google hasn't made their E2EE extensions available outside their app. Where's your "there's no excuse" outrage?
You're trying to excoriate Apple for not supporting an extremely poorly implemented "standard" (RCS) or investing tons of money fighting with carriers and partners trying to create some new "standard". It's absurd.
If Apple tried to support the RCS clusterfuck that would be a third protocol supported in their Messages app. Even if it was some new standard hammered out with Google and carriers there would still be an SMS/MMS fallback for the literal billions of handsets that won't support whatever new standard.