The problem with making the Chat Moss line the only route for through expresses, means that Liverpool South Parkway would no longer be served by fast (or even through slow) trains to Manchester. A significant proportion of the passengers from Parkway rely on those trains for commuting or longer distance travel, and it would seriously add to their travel time and inconvenience if they were forced to travel via Lime Street. Parkway is used by many residents of affluent south Liverpool, who would probably rethink their travel habits and revert to cars if that option was blocked.
This is obviously a risk of performing this converison, but we can get an idea of the scope of the problem by taking a look at the 21/22 Destination Matrix.
There are 718,189 journeys recorded from Liverpool South Parkway to the 172 stations that had more than one hundred journeys.
558,083 of those journeys are to stations on the Merseyrail network and thus don't really matter for this discussion. I know that's a bit of an iffy assumption for Liverpool Lime Street but the fast trains on the CLC only form a portion of the fast trains to Lime Street from South Parkway, so I think its defensible.
A further 19798 journeys are to stations on the CLC from Warrington Central to the West. So those would be joining the Merseyrail network, so they are not really relevant either.
That leaves us with 140,320 journeys to other stations.
Unsurprisingly the top entries are Oxford Road and Piccadilly, with the Manchester station group making up ~58,000 of the remaining passengers.
This is significant, no doubt about it, but it is still only a tiny fraction of the journeys made from Liverpool South Parkway.
The next three entries that make up the top five of the "Other" categories are Runcorn, Crewe and Birmingham New Street. A lot of the Other stations seem to be stations towards Birmingham and London, which I guess is not surprising.
There will be losses to stations like Sheffield et al from the Liverpool-Norwich trains, but the extra inconvenience will be far less important there.
I think on balance that the conversion is likely to result in major increases in net traffic, none of these flows are particularly large compared to the major increase in patronage we would expect on the line to Warrington from Merseyrail conversion.
Extending Merseyrail to Warrington Central should not present a huge challenge, there's no reason why the 'slow' trains offered by Merseyrail would need to be segregated from the fast trains. Having the 'slow' train from Oxford Road and the 'slow' train from Liverpool both terminate at Warrington Central should not present too much of a challenge. Although it would have an impact on the journey opportunities from Warrington West unless the 'fast' trains start calling there.
That would result in Merseyrail losing its primarily operational advantage and is unlikely to be popular with the train planners.
For the first time, there would be major exposure to delays introduced from outside the Merseyrail system.
Crossing the CLC at Hunt's Cross is one thing, but travelling for miles through multiple station stops and then terminating in shared platforms at Warrington Central is quite another. There isn't even really room for a bay at Warrington Central so you'd have four trains per hour terminating in the same two platforms as are used by fast trains.
The biggest challenge, IMO, would be working out what to do with West Allerton and Mossley Hill.
I'd suggest they would have to be picked up by the Liverpool-Birmingham trains operated by LNWR.
Extending Metrolink to Warrington would be dumb though. Keeping the mix of 'fast' and 'slow' trains makes sense.
If, as is likely, you have to put in buffer stops at Warrington, is there any point in keeping fast services to Warrington Central when Warrington already has sub-30 minute trains to Manchester from Bank Quay?
