Easier for ATC to enforce, yes, but far less accurate of course.
At the moment there are loads of variables for regulated flights: Airborne within a 15 minute window, and then a flight of up to two or three hours during which winds, route, airspeed might all change.
Add to that list taxi time (at Heathrow this might be as little as 5 minutes, to as much as an hour), and the effectiveness of putting a regulation on is almost zero.