The reason MAN has unused runway capacity is because all the airlines that believe they can make money on routes from there are already doing so.
That observation has some merit, but might be better rephrased as "Some major Eurpoean-based groups do not believe that they can make money on routes from anywhere in the UK other than London".
Manchester seems to have a growing number of airlines offering long-haul services both east and west, with rapidly increasing premium cabin capacity to a range of destinations. Other airports outside the south-east are seeing similar growth, though perhaps not on the scale of Manchester.