It's simply wrong to infer that IFM's interest is solely in Stansted. Their investment is in the MAG group as a whole, and the non-STN airports will continue to generate the bulk of group profits for the foreseeable future.
Some confusion may have arisen over the fact that IFM's investment in MAG was conditional on winning Stansted. But this was at the insistence of MAG and its local authority shareholders, not IFM. The point is that MAG only needed the capital provided by a new shareholder in the event of winning Stansted. Without Stansted there would have been no need for new capital.