* Apple is really popular in the US
* Apple users tend to rely heavily on Apple's default applications
* Apple's messaging app is the default, and works fine with other Apple devices, but sends shitty SMS or MMS to non-Apple devices
SMS would disappear tomorrow if Apple adopts RCS.
And if they allowed iMessage clients on other platforms, they could corner the entire messaging market.
Once you automatically reject SMSes from those contacts, such that you don’t even know they're trying to contact you, the ball is entirely in their park to take action.
And it's not like there' some gigantic combinatorial explosion of apps you have to install. The vast majority of messaging around the world is about 5 apps. Facebook's Messager, Wechat, Instagram, Whatsapp, Discord. Between these, you'll reach the vast majority of the world's population somehow. And then you'll need one or two more locally-used ones like LINE or KakaoTalk depending who you're talking to.