Don't get me wrong, I agree with everything you've said here.
SMS and MMS are an antique technology, I can't argue that. But it's not going to change the fact that my family is not going to install another messaging app on their device in order to talk to me. They're going to text me or call me anyway so therefore there's no point, and hopefully with the improvements on the RCS standard the issues that have occurred with SMS and MMS will go away.
As is I have four different messaging apps on my phone ignoring my messages app, signal (which I can count on one hand the amount of my friends that have an account), Discord which the majority of my online friends are on, and less than a handful of my relatives are on. Telegram which I mostly have for artists, and Revolt which I really should uninstall but like I really want that project to go somewhere.
My family is almost exclusively on Facebook messenger, I do not use Facebook Messenger, sms/rcs is the only system that my relatives and I both have, and they're not about to install another app, to talk to one person which would be me.
So yes I fully agree with everything you've stated there, 100%. But it's a perfect example of how on paper it sounds amazing but in practice it doesn't work. At the end of the day my family is going to text me regardless if I tell them that I'm on Signal, because I'm not on Facebook and they already have SMS on their phone