Pemberton and Orrel are both rather low usage stations compared to those found on the Merseyrail system, even before coronavirus Pemberton managed only ~70,000 whilst Orrel about ~95,000. Neither station has recovered to its pre-coronavirus values.
If those stations were on Merseyrail they would be amongst the quietest on the system. Which is not surprising considering the comparatively poor service that they receive.
The 2021-22 Origin-Destination matrix is from the highly reduced demand period of coronavirus, but it is also the most up to date available.
For Pemberton, 28 destination stations had a hundred journeys or more, but the top fifteen correspond to an enormous fraction of the total journeys.
- Manchester Piccadilly (3653)
- Wigan Wallgate (3164)
- Wigan North Western (1718)
- Liverpool Lime Street (1187)
- Manchester Victoria (1175)
- Liverpool Central (987)
- Kirkby (Merseyside) (964)
- Salford Central (760)
- Manchester Oxford Road (751)
- Salford Crescent (647)
- Upholland (427)
- Rainford (382)
- Hindley (304)
- Moorfields (276)
- Daisy Hill (265)
Wigan stations, Upholland, Rainford and Kirkby (at this time) are on the branch, so the only impact of moving the terminus would be an effective increase in frequency. They are a major traffic source, with 6655 journeys.
7555 journeys are listed as being to the Manchester Stations or to Daisy Hill/Hindley (I think the split is a function of the Manchester Stations Group more than anything).
The Merseyrail journeys (excluding Kirkby) are 2450 journeys total.
So we have an improvement for 2450 + 6655, and a loss of direct trains for 7555 journeys. 9105 vs 7555. Obviously split ticketing will have an impact but both Wigan and Kirkby would benefit from split-ticketing. This station is the most Manchester-centric on the branch, with the other extreme being Rainford, where Manchester Piccadilly is the most popular non-Merseyrail or branch destination at number 6!
Even at Upholland, Manchester Piccadilly is number 3 and Liverpool and Branch destinations dominate.
Sure, imposing a change at Wigan might lose the railway some customers from Manchester stations upset by the change of train. However, the destination split is surprisingly Liverpool focussed at all but one of the stations on the branch. Give the low traffic levels achieved today, I don't think there can be much doubt that extending Merseyrail to Wigan Wallgate and cutting back the Northern service accordingly would result in a major increase in traffic levels.
Railways are bulk public transport systems, and Merseyrail levels of traffic are what we should be aiming for, a 1tph rail branch does not achieve the primary purpose of the railway, to mvoe large numbers of people. Plus cutting the service on the branch from the east end would free a Manchester path for Southport.
EDIT:
It's not even clear to me that 1tph direct is actually better for users than 4tph to Wigan and then 4tph to Manchester from Wallgate and more from North Western