you would have to opt in. And presumably if they have to pay you a $250 voucher, the person buying your seat would be paying $250+ more for your seat that you were. That's probably fairly rare
No, it isn't. Ticket prices fluctuate incredibly wildly, and tend to increase sharply in the month leading up to the flight. In fact, this move is largely to displace overbooking to a new mechanic.
Now why this is a problem: Want to book a necessary flight 3 months ahead of time, shop for the cheapest fare, amd then find out a week before your trip that you've been bumped to a later flight (posssibly missing scheduled activities) because a last minute customer wants your seat? Unoversal adoption of United's model would make this common place (at least as common-place as overbooking is now).