I don't the % of traffic carried by Chiltern is actually that high. Although trains are well loaded, a good chunk is to the likes of Bicester and Banbury. I was on the 1815 off Marylebone a few weeks back, and I was only one of maybe 3 or 4 getting off at Snow Hill.
The easy capacity fixes have by and large been done. The extra capacity is being provided to meet demand at the busiest times of day.
The £10 tickets are to fill this capacity at the times of day it is not required. Exactly like today.
London Overground and Bakerloo line are half length.
Hull and Grand Central ditto.
Virgin Trains fleet is still a large part 9 cars.
East Midlands Meridians have very low capacity.
Chiltern aren't full length either.
3 paths for maritime shipping container trains north on the WCML in the evening peak.
LNER Azumas coming soon...