Because there are costs associated with keeping the plan around that you're not privy to. Specifically, what is it that you think you're getting that they both can't tell you about and couldn't change without you knowing? It's not price, so it must be speed or throttling/deprioritization, any of which should be remarkably easy to prove.
You kinda act like there was some Super Secret feature unlocked on your phone the day you signed up for the old UDP, and that they can't just change it on a whim. The same verbiage that allows them to change the price allows them to change features or discontinue the plan at their pleasure.