... remembering that a 5x810 has 12 more standard class seats than a 7x222 and 60 more than a 5x222, so with the exception of where a 5x810 replaces a 7x222, passengers are going to see a significant capacity increase (although of course the thinning out of the EMR intercity fleet means some of these may have been downgraded recently).
Yes, surely the reasoning behind the 810s as 5x24m units has surely been done to death?
The reality is that the alternative to a fleet of solely 5x24m units is a progressively smaller number of 10 car trains, but as you lose flexibility you need more carriages. e.g. running all EMR IC services as 10-car sets needs about 20 diagrams, so perhaps 24 units, which means 240 carriages need paying for, rather than 165. Good luck taking that proposal to Mark Harper.