Your passenger stats of WCML growth to and from Manchester might be easier to accept as proof of the need for extra capacity if you also showed the current capacity for each hour of the day. I suspect that many Virgin trains have plenty of capacity.
That information isn't in the public domain, however if we take the 11.2 million divided that by 360 days, 15 hours a day, 2 directions of travel and 5 trains an hour your have 200 passengers per train, on each and every train, making the trip between London and the North West.
However those services stop at other locations and early morning/late evening the number of services drop off a bit.
As such although that's an average load of ~35-43% that's just part of the picture, as there will also be those traveling shorter distances which will be on top of that. It also doesn't take into account any tidal flow in traffic (more arriving than departing in the morning peak and vice versa).
As such chances are there a good few services which see loadings of double that ( 70-86%).
The other problem is with doing that is that is what it looks like today, not what it looks like in 10 years time.
At 2.5% growth that could well be an average 55% loading just on the London Manchester passengers.
However last year there was 3.1% growth on Virgin services, if that continued for 10 years and it's approaching 60% full before you consider the local passenger flows or tidal flows or that the earlier/later services tend to be a bit quieter. At that point you'll see even more stations cut from the long distance services to facilitate it.