It's a difficult one isn't it. The fact that XC sevices are:
a) Very expensive
b) Very busy
c) Losing money
Suggests to me that something within XC is simply not fit for purpose. I can't help thinking the massive lack of capacity causing both overcrowding and preventing the availability of generally good value fares as per say Virgin Trains or FGW is at the heart of this.
To resolve c (to a certain amount) and given b the simple answer would be to have better length trains running XC services. If all trains were lengthed to 7 coaches there would proberbly be very few empty seats over the majority of their services. The problem is there is no cheap way of doing that (it was one reason why project Thor was looked forward by so many).
Yes, some of their costs would go up (track access charges, leasing and fuel), however their staffing and head office costs would stay the same whist their income from fares would go up, proberbly by more than the costs which went up. In reallity the cost per seat to run a train would go down, meaning that as long as they could sell a few extra tickets to cover the extra costs them.
For example if an extra coach cost £1,000 per day, it wouldn't take many extra tickets at £20, £50 and £100 to cover those extra costs. As an example if they could just need to sell 2x£100, 8x£50 and 20x£20, or 30 extra tickets for a coach with 70 seats. However that isn't about 40% of capacity of the coach, as many of those passengers could be using the same seat. Passenger A boards in Bournmouth and leaves at Reading, whose seat is then used by passenger B who then travels to Manchester.
Now it is unlikely that passenger income would increase enough from day one to cover the extra costs, but over 6 to 12 months it could well do enough that it is worth doing. The problem is that it would require some extra subsidy for a while for someone to do.