(I'm in the EU, and I haven't seen a phone sold here (in a shop) that wasn't compatible with all available providers - to be honest, this problem has never crossed my mind when I bought a phone.)
2G is being shutdown too (IIRC, I had a 2G pay-as-you-go phone that stopped working years ago), and it would be surprising to me that a carrier would shutdown 3G before 2G.
Also, updates to SMS implementations eventually broke SMS on the good network.
So I switched to the Punkt MP02. It has a lot of other problems, though, but at least those are more passable.
I did also try the Mudita Pure. But it froze on the second day I had it, so I switched back. Edit: My plan is to give the Mudita Pure a fair shot in the future.
I'm generally disappointed in the phone market, smartphone and dumb phone. Maybe I should abandon it entirely and get a landline.
That is one expensive dumb phone.