Paxboy, this is always a possibility because private operators naturally want to maximise profits for their shareholders.
However if a third runway is railroaded through...oh, be quick, look at that flock of pigs!...there are likely to be conditions, one of which would be a raising of the limit of 480,000 movements.
If this limit is set at a lower figure than 720,000, with a third runway, the airport can be required to operate at an optimal operational capacity (say 80%) rather than an optimally profitable one (100%), whilst keeping alternation on the two primary runways and the overnight closure.
That said, there is still plenty of scope for expansion and the owners can still make money.