The best way of stimulating demand is to increase frequency. The railway has significant fixed costs which apply regardless of the level of traffic, so in the long term the best thing to do is to take a short term hit on profitability/subsidy levels to stimulate traffic growth. Making significant reductions in subsidy levels, or increases in profit, is much easier from that point on.
Manchester has another option, which is to provide that extra frequency using Metrolink rather than Northern. Ramping up frequencies is much cheaper on Metrolink since everything is cheaper - the vehicles, the staffing, and most importantly the infrastructure. Adding another 10tph through Victoria or the Castlefield corridor would cost a lot of money, but you could probably get away without any real changes on the street-running section. In the worse case scenario you just need to build a Third City Crossing, which would then allow higher frequencies on other profitable branches so the financial impact wouldn't be too bad. Then, with the released capacity on the NR infrastructure, you can optimise the heavy rail services for revenue maximisation.