Didn’t realise XC made a profit!
Making money out of adding capacity depends on there being significant demand that is currently put off. I am not saying there isn’t but you lose money if you add a coach and only carry the same people (but they all get a seat). There is also the issue of providing the extra capacity for the whole diagram but it might only be crowded for a small part in the middle.
Whilst the extra coach would only mean extra seats just to absorb those who are already standing you'll then attract more people to stand.
However that misses the point that there would be a load of extra seats for those services which have gone from a single 4 coach unit to a pair of 5 coach units over the core.
! They're extremely reliable and I understand they had a refresh in 2016/2017 - I know because XC said.