I don't use SMS myself but in this case it sounds like I'd be better off just not being your friend.

Sounds like you'd prefer to keep inflicting to me and to yourself a degraded experience rather than making the tiny, one-time effort of installing a free app. Because that's the whole point of this issue: the fact that you can still get what you want (reaching me) is what prevents you from making the smallest effort to make both our lives better and easier. And I also don't expect my friends to behave like that.

Is this a legitimate question? No one is going to download an app and use it to message one guy.

No one? I did. Normal, if you really care about that guy. In any case, the app is free, what does it cost you? Plus, the more people do it, the easier is for everyone to move to an app that works for everyone.

Yes, it's a legitimate question. If you wanted to keep up with a friend from Japan, would you not install LINE to talk to them (or them installing Whatsapp or Discord, or whatever you are using)?

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.