I know the promise was to never raise the price, but I filed that under "too good to be true." Prices pretty much always go up on everything.
If they've promised - in writing - to maintain a price forever, they are legally obligated to offer that price forever. If they want to increase prices, they need to increase them on new customers. You shouldn't need to be 'bummed' about paying more, because you shouldn't be paying more.
My gym gave me a 'foundation membership' which locked in the price I paid when I signed up 15 years ago. They recently tried to increase it to double that amount, and I pushed back. They ended up honoring the original price, because that's the entire point of a foundation membership.
A company can't say 'Buy one get one FREE*' and then put in small print that the *company has the right to charge full price for the free item, if you don't wish to pay full price for the free item, you can cancel your purchase after buying one. What T-Mobile has done is effectively this, but with a price increase on the existing service, rather than an additional free item. It's misleading and deceptive.
In Australia, a company can be fined up to 10% of their annual revenue for engaging in this kind of behavior, as well as having to compensate customers and honor the original promise.