You definitely shouldn't invest time in SMS. Without RCS (or some custom messaging protocol support), "texting apps" are pretty much a dead market.
RCS is both more secure and more user friendly than SMS can be by design. Once the iPhone gets RCS support in the coming months/years, this will be especially true.
If you message cross-platform from Android to iOS, and you can't get everyone to agree upon a 3rd party app, then you're kind of stuck with sms. This isn't a problem that is going away in the near future. Apple relies on their locked messaging platform to influence their users into thinking iOS is the best. The users then pressure all of their friends to get iPhones too. It's an effective strategy for them. Very few iPhone users seem to understand the games being played.
In fairness, even the author of the article you linked didn't think that'd ever happen. I'm somewhat up to date on most general tech things and missed this announcement too (though to be honest I don't follow Apple announcements anyway).
Plenty of services still unfortunately rely on SMS for 2FA, so we'll still need a client to receive them. Doesn't really need to be able to send them though, I don't think any human has sent an SMS message deliberately for about 12 years!
I don’t know anyone in Canada that isn’t a new Canadian that uses anything other than the default sms app on their phone. (FBM and Insta and what not don’t count in my example).
I don't know about the underlying technology, but every client I've used for the past couple decades supported groups.
However, they absolutely sucked. There was no way to leave. Unless someone made a new group without you and everyone used that, you'd keep getting messages.
More seriously, in these cases, I often rely on emails instead. But I don't really use group chats a lot (even with my contacts that are on Signal), nor send many files, so it's not features I really miss. And SMS works with everyone, so it's always my default if the contact isn't on Signal.
Group chats work over SMS. I use cloud links for large files or just SMS/MMS for pictures and you just live with the low quality if texting with an iPhone user.
You're right, I've updated my reply... Hopefully someday that will be a generic RCS feature not exclusive to Google and/or Apple and Google will work to make the E2EE work between their two systems.