You keep suggesting this...
...but put yourself in the ROSCOs position. Say they have a hundred coaches to lease.
You want to only lease ninety, leaving ten end cars unwanted. That then costs the ROSCO more money in storage etc, since they have an unwanted asset (nobody is going to hire just the end cars, so they are essentially going to be a drain on the ROSCO resources - still of some value since they could be used in future if the trains are reformed - they aren't going to just scrap them straight away).
So leasing ninety carriages might cost more than leasing a hundred - the ROSCO don't want to be lumbered with the remainder.
However on the other side of the coin you have the TOC's who don't want to lease a coach which has 1/2 the number of seats in our and then have to have extra staff to run the services as you need staff in both parts of the train.
That then means that you've got a choice go for a new train, quite possibly an 80x which would also save you money in fuel costs as well as giving you a higher capacity unit for the same number of coaches, or have an older train which could have higher maintenance costs (case in point the 1/3 saving in maintained costs on the Desiro City fort over the original).
Even then there's the fact that is you are refurbishing the 22x fleet not having to do 10% of your fleet keeps those costs down. There's also nothing stopping you scraping those end coaches as at the very least the end coaches from the 4 coach units wouldn't be missed. However, there's a good chance that the 22x's are going to be circa 30 years old when they get dropped from the next lease. That's not that far off the typically accepted age of a DMU of 35. As such if they all get scrapped at that point that's not really going to cause much of a problem of you then scrap a few middle coaches early.
Anyway if you don't lease any of them that's 100 coaches to store and not 10 with 90 earning you an income.