My point is other operators, most others can justify the expense, GWR use them on services which can afford new stock and TfW don't plan on using them long term. Northern has a very small fleet of only 8 and probably won't get more as although there is some electrification, it isn't enough and as others said, Northern doesn't have much budget for trains.
The thing is though that the cost of trains is typically broadly split into quarters and so if you double the number of coaches on a service you doubt double the costs, as 28% of the costs are staff costs which remain the same whether it's 1 coach or 12 coaches.
Whilst in comparison 20% of costs are rolling stock costs, which will change significantly as you change the length of your trains.
The other 52% is made up of track access, delay payments, and other costs (including HQ and website costs). Again a lot of this is unlikely to change much if you are running more services, especially if the trains are more reliable.
As such if you were to double the total number of coaches you have you'd only need to generate something like 1/3 more income (maybe up to around 50%) to cover those extra costs.
Given that typically 45% of tickets are season tickets and how busy peak hour services are then there's a good chance that you could not need much more subsidy if you were to significantly increase the number of trains running and in time (as passenger numbers grow) it would likely reduce the subsidy required.
Especially if you can make use of more EMU's which historically used to be 9% cheaper to lease, although I would guess that with a lot of EMU's off lease the cost difference is likely to be more.