Stagecoach running EMT already do what you are afraid Arriva might do (and what XC do) if they got the EMT franchise - i.e. ruunning short trains and keeping longer trains in the sidings. The sidings at Cricklewood are full of HST's and five and seven car Meridians for much of the day during weekdays, yet passengers are crammed in to four or five car Meridians on the off peak semi fast St Pancras - Nottingham/Sheffield services, which are often full and standing. It is even worse on a Sunday when there is only one train an hour to Nottingham and Sheffield for much of the day covering all stops and often only formed or four or five car Meridians, resulting in some really appalling overcrowding. So I can't really see how much worse it could get under Arriva (or anyone else). Yes, I know it's all about money and cutting down on leasing costs etc.. but it's terrible customer service and passengers using EMT have been getting a raw deal under Stagecoach for many years. Will be glad to see the back of them.
I've seen Northern do this a fair few times.