The usual gotcha is you can keep the plan if you want, but slowly the plan becomes unusable. Imagine having a very cheap unlimited 2G-only plan in a world where even 3G networks have been shut down.

I have a $23.49/month unlimited AT&T hotspot plan that will die once the device it is locked to dies.

Right. I don't blame T-Mobile for raising prices per se, it's not really an unreasonable increase. But they were idiots to ever promise that they would never raise prices. It's not possible to uphold that deal and still make money.

they didn't really promise that they would never raise prices, they promised that if they did, they would cover your last bill (which people are also having a hard time for them to fulfill).

Because improvements in technology would decrease expenses at a rate faster than inflation.

Im guessing that was the idea.

To get customers now, even though you know you will break the promise in the future.

Perhaps if they expected a Moore's Law of some sort to apply to bandwidth costs? That is, they expected expense of maintaining a basic plan to drop over time far faster than inflation.