Why did I get a text about an increase for the One plan?

I'm still not fully convinced they can do this even if they are ok with the  reputational damage that comes with it. My clear memory of signing up for the One plan in 2018 was that it could never change, that "T-Mobile was signing on the dotted line" and all this weasel-wording about just paying your last month if they break the promise must have come way later and shouldn't be enforceable for those older plans. The original un-contract language even says, in part,

“

¿PUEDE CAMBIAR O TERMINAR MIS SERVICIOS O ESTE CONTRATO T-MOBILE?

Sí. Excepto como está descrito a continuación para Planes Tarifarios con garantías de precio fijo (incluyendo la "Promesa sin Contrato"),

…

If you are on a price-lock guaranteed Rate Plan, we will not increase your monthly recurring Service charge (“Recurring Charge”) for the period that applies to your Rate Plan, or, if no specific period applies, for as long as you continuously remain a customer in good standing on a qualifying Rate Plan.

“

Which appears to bind T-Mobile to the original terms.