“T‑Mobile will never change the price you pay for your T‑Mobile ONE plan.” That was the promise. The Un-contract. The whole reason millions of customers picked the magenta team over Verizon and AT&T in the first place. Now T-Mobile is retiring legacy 3G and 4G-era plans — Magenta, ONE, Simple Choice — and automatically moving customers onto “modern” 5G plans at higher monthly costs. Billing changes hit mid-July for the current wave. The company that swore it would never surprise you with a rate hike just sent the notification.
Switched to an MVNO and have been quite happy with it. Cut bill by almost 2/3.
MVNOs do have a lower data priority than the parent network, but IME this hasn’t made much difference. I don’t tend to stream a lot of video on my phone though, so it might be a concern if you need fast data.