I got the same text today. Rates raised by $6 per line. I sort of remember that the original rate was no contract and 'guaranteed for life', but haven't found where that was actually promised. I've been on the ONE Plan Unlimited 55 for years.
The only wording I've found says 'Those were our commitments that only you have the power to change what you pay for your monthly talk, text, and data rate plan. We will pay your final month's recurring service charge if we raise prices and you choose to leave.' So, not much of a guarantee - if we raise the price, you can leave with this parting gift.
But - the T-Mobile plan is valuable to me for the unlimited overseas text and data. It's worked pretty seamlessly in every country that I've been to. So I supposed I'll have to swallow the increase.