I can opt out, but then any streaming goes against my high speed data limit.

I think it’s actually a pretty fair trade off.

Even if this were technically feasible, it sounds like a massive infrastructure investment with little to no value. T-Mobile would need to have enough compute and network horsepower to DPI all outbound traffic, intercept every video stream, detect if it's over some resolution threshold, and re-encode it at a lower resolution or bit rate, all in real time.