My point was also about new growth in the market, the London Manchester market - which I believe is massively suppressed through super high fares.
Is the most efficient way of growing that market really to run a trainload of London-Manchester passengers, half of which (optimistic estimate) would've otherwise not travelled by train, through the Manchester-Stoke line and MML every other hour? The MML could do with enough growth as it is (more trains to Derby/Nottingham would fill up quickly), same with the line through Stoke and Macclesfield, but if you run a train that's full of Manchester passengers on this route, you're denying GBR/EMR the chance of doing that in the future.
If you do want to encourage more rail travel between London and Manchester, the best way is to create more capacity, through HS2, and also look at fairer pricing models than the current scenario where you can either attempt your luck at the advance ticket lottery, or pay up £200 for a walk-up fare. In the meantime, attempted growth between London and Manchester via alternatives methods (such as OAOs via the MML) will harm growth elsewhere.
Why can't that be explored for Manchester, when it has for Edinburgh? Which is an identical route.
It was possible to fit in extra London-Edinburgh paths on the ECML, on a fast route that's even faster than LNER. GC takes over 1h longer than Avanti here.
The WCML is entirely full - adding any fast Manchester trains will require cutting other services.
If you want to compare it with Edinburgh, imagine an OAO service from St Pancras to Edinburgh via Sheffield. Would certainly allow for more London-Edinburgh growth, at the expense of most other stations on the line.
A lot of stations on GC's proposed route that really aren't reaching their potential - such as Luton Airport Parkway and Wellingborough (no trains to the Midlands) or Congleton and Uttoxeter (sizeable towns with an underwhelming service level), but if you check the planned timetable, you'll realise that they don't even plan to stop all trains there. I'd guess that there won't be a lot of competitive fares to/from these places (compared to the number of London-Leicester, London-Stoke or London-Manchester fares) either.
It's painstakingly obvious that London-Manchester traffic is #1 priority for GC here and everything else is just done to convince the ORR to approve the application - it isn't that encouraging new travellers onto the railway isn't an admirable thing, but you do need to consider the consequences of it as well. Railway lines in Italy or France where there's a lot of spare capacity on a segregated fast line that the state is underutilising are a different bag to mixed-use capacity-constrained railways like the MML.