Pax are going to love this policy, right up until they sit on the tarmac for 90 minutes, then get told that their flight is cancelled because the airline isn't willing to risk the fine. I *promise* you that if UA (or anyone else) thinks there is even a 5% chance that they won't make the cutoff window, that airplane is going back to the gate and then they'll cancel it, because risking another 2 hour taxi delay throws the aircraft and crew scheduling into chaos.

Long term this rule may actually bring about sanity in the way that flights are scheduled and over-booked (especially peak times at busy airports) but in the short term, it's gonna be a total clusterfork.

Be very careful what you wish for.