There is capacity overall in the NW. If people want to fly, they'll find a route somewhere that meets their needs of convenience and cost.
If airlines find demand for a route pair, they'll supply a route from a NW airport to meet it.
MAN is a mature airport as recent pax figures show. Any capacity for new routes/pax will be at a higher marginal cost, and lower marginal efficiency/convenience/ease of use.
It is more likely that other NW airports with capacity to provide it to airlines and pax at a lower marginal cost & a greater marginal benefit will grow to increase overall NW capacity.
That's the free market. It's only political influencing that distorts it; something MA PLC has always been good at.