as an unwilling Whatsapp user the ability to migrate without having to convince all my social circles to do anything but check a checkbox sounds like a huge step forward.

That's the point. I feel it will not be a "simple checkbox", and they will make it the most obnoxious process they can using the Best Dark Patterns the industry has to offer.

Already the general public is not interested in the alternatives or the concept of interoperability - wanting something that Just Works™ - putting in front even the smallest step (and some scary text!) will make the percentage of willing people become even lower.
And that's not all. As it is portraited in the article by the Threema's spokeperson it is pretty clear that Meta will just try to make the maintenance of the communication layer as cumbersome as they can - both technically and bureaucratically.
They are explicitly the ones keeping the reins of the standard, the features, the security model, the exchanged data and who, how and when will be approved.

So from one side if they make it hard and scary enough to tank the use rate, they will have the excuse of not being there enough people to give priority to fix it or add features, and from the other side if maintaining the interoperability will be difficult and time consuming enough, the people and businesses from the alternatives or wrappers will not have the incentive to do or keep doing it for the long haul. As we can already see in the article.

Is it better than nothing? Sure, probably. Will it be a slow cooking, easy to break, easy to get excluded from, just bare minimum to comply to the letter but not the spirit of the law? I feel that's a pretty good bet to make.

Let's be clear: I will be extremely happy if all the red flags and warning bells that I saw in the article will just end up being figments of my imagination. But yes, I'm very pessimistic - maybe even too much - when I see these kind of corporate speech and keywords.

No, they have to convince everyone they talk to to enable the setting to be able to use this

Yeah lol. And it's not as if boomers don't get scammed enough already.

Google weeps in corner

From what I understand, it's implemented on mobile carrier level, so any phone or other device with a cellular modem connected to a carrier that supports it should be able to use it (edit: of course, ignoring insufficient access, at the very least rooted Android or some Linux should work). Can't really find more specific details right now though. Here's a library and sample client for it though: https://github.com/Hirohumi/rust-rcs-client

Of course, this is only for clients, it's true you can't set up your own server.