You sure? AFAIK Northern operate one service from Manchester to Blackpool each hour with one more to Preston (from Hazel Grove), however I believe there are to be three services per hour over the Moss (currently one goes to Warrington Bank Quay off-peak) and two per hour from Liverpool to Wigan, not forgetting the service from Liverpool South Parkway to Blackpool each hour that might also benefit from electric trains, I make that a quarter of services.
Services on Bolton corridor that will switch to EMU:
* Hazel Grove to Preston
* Manchester Victoria to Blackpool
* Manchester Airport to Blackpool*
Services on Chat Moss that will switch to EMU:
* Liverpool to Wigan
* Liverpool South Parkway to Blackpool
* Liverpool to Manchester/Stalybridge
* Liverpool to Manchester Airport*
* The future of these services in the long term is uncertain. Blackpool to Manchester Airport may or may not transfer to Northern and may or may not get 319s. Liverpool to Manchester Airport may not continue to operate on Chat Moss.
So that's 43% Bolton and 57% Chat Moss, so 'around half' isn't a bad approximation for what you earlier referred to as 'only one route.'
OK Liverpool to Manchester/Stalybridge is likely to be 2tph but there are plans for additional local services between Bolton and Manchester once the Scottish services are diverted away from Bolton, so there may be more on the Bolton corridor as well.
And give Northern (or whomever operates the service by then) another class of train to do maintenance on. A standard fleet of 319s would give more operational flexibility at no additional cost.
Note that the long term future of the 323s in the North West isn't confirmed. That was something that came up when TfGM looked in to funding the CCTV. The agreement drawn up is that the long term future of the 323s in Greater Manchester is not guaranteed but if they are replaced then the replacement EMUs will already have CCTV or have it fitted at no expense to TfGM.