I doubt that they'll want to change the $30/mo + taxes/fees you're paying for "unlimited" (really 35GB) service. T-Mobile already offers a $40/mo unlimited plan on Metro with taxes and fees included and that's without the annual pre-payment. As I noted in another comment, Mint values the annual pre-payment at over $10/mo compared to quarterly payments. I think it's reasonable to say that $360/year ($30/mo) seems roughly equivalent to $40-45/mo on a monthly basis. Plus, taxes and fees increase the price of the plan such that it's probably in the $35-40/mo range.
Visible is already offering $25/mo unlimited service (on promotion down from $30/mo) without annual pre-payment via a similar low-touch model (and owned by Verizon) and Visible doesn't charge taxes/fees. It seems unlikely that T-Mobile would want to raise prices on your plan when you're already paying for a year in advance and you're already paying 30-60% more than Visible is charging and Visible doesn't even require annual pre-payment for that rate.
> I am guessing it will soon be back to $85.
This is literally FUD. T-Mobile already offers $40 unlimited (with taxes/fees included) on a higher-touch Metro plan that doesn't require annual pre-payment. Even a single line T-Mobile Magenta plan is $70/mo. This sentence is simply FUD.
You might like your deal, but it's not that great a deal by objective standards. Visible is cheaper and doesn't require annual pre-payment.
This is T-Mobile wanting a low-touch brand similar to Visible (which would be even cheaper for your if Verizon works well in your area).