LM won't have any capacity robbed off them if they retain the same number of Turbostar carriages and 170s also have nicer seating than 172s, so there would no disadvantage for LM passengers as a result of my suggested swap. This is very different to TPE getting 156s (which should have been providing extra capacity on Northern services) to replace Chiltern bound 170s and it also being likely some Northern services will be 142/150s from May instead of 156s.
I think that you've missed the point of all the horse trading that goes on between ROSCOs and TOCs under the watch of the DfT and their predecessors. The rolling stock part of the UK's railways since 'privatisation' has been all about three things:
1) to weaken the rail unions that operated when there was a single employer (BR)
2) to sell-off a national asset to business
3) to allow various commercial enterprises to generate profit from operating in a quasi-competitive non state-run railway
Passengers are just pawns in this game. There are no ethical values in the processes, except when there is an opportunity for the government of the day to score points with their intended voters. So arguments about the relative running costs of different train classes, how comfortable the seats are for certain types of service, whether one franchise (or it's passengers) have any presumed entitlement to certain types of stock etc. are just lost in the noise of maximising large international groups' profits.
The DfT are complicit in this farrago in protecting it's survival and when necessary jumping to the government of the day's political needs.
There, that's got it off my chest. You must believe that I honestly don't find the above anything less than scandalous, but this is the way it has been since the first privatisers in the '80s got that glint in their collective eyes.
So in my view, terms like 'robbing' and 'ethical' are not part of the process.