It works for at least one mobile carrier, as demonstrated in the article Krebs references [1]. One carrier is enough.

The ability to hijack text messages through an online service was also shown in an article Krebs' source mentioned [2].

Perhaps you're right that upstream agreements with mobile carriers should already be blocked, but in practice it's been proven not to be.

There was a change to NetNumber's systems on 11 March to combat this, but there's no guarantee that their competitors don't have similar flaws or that the measures taken were good enough to stop the attack in practice.

Even still, if the problem might be fixed in the entire US, that doesn't mean anything for the rest of the world.

[1]: https://www.vice.com/en/article/y3g8wb/hacker-got-my-texts-1...

[2]: https://lucky225.medium.com/its-time-to-stop-using-sms-for-a...