If you could predict the future that might work. But what if the 142 got to Warrington, and then the line reopened? You're left with the first train from there to Chester and beyond being a 142, and even more of a mess to fix in terms of sets being in the wrong place.
Well ATW (presumably after consulting with Network Rail or emergency services) announced before 5pm that the Chester-Warrington line would not reopen until after 7pm, so presumably they based their emergency planning around that. For instance, it's been mentioned already that they sent a Crewe driver up to Warrington so that they could get the loco-hauled set back where it should be.
You could argue it's more of a mess having passengers trying to use an alternative service out of Manchester in the evening peak, when that service is usually an overcrowded 2 car Pacer even without any extra passengers, than it would be to split what's advertised as a through service at an intermediate station if things return to normal sooner than expected. Incidentally, Chester does finish up with a surplus of both Northern and ATW units during the evening peak, with both running extra trains to Chester but not from Chester.
As far as train crew are concerned, in an ideal joined up railway you could possibly make a case for Chester crews signing 142's so they could work the Northern Chester-Manchester services at the extreme ends of the day that currently rely on inefficient ECS and taxi journeys (like they did when it was all one TOC), but even if you did you'd never have a case for training Holyhead crews on them.
When the North Wales routes were originally transferred there were agreements in place to reduce such inefficiencies and there were services on the Warrington line worked by FNW crews and/or units and ones on the Mid-Cheshire line worked by W&B crews and/or units.
I recall a crew member in Arriva uniform working a 142 from Manchester to Chester via Altrincham one day late 2003/early 2004 when a passenger asked if it was the train for Chester. He responded telling him not to travel on it if he was going to Chester as it's a very old train that takes an indirect route.
Then Northern came along. Apparently there were talks about continuing to work each others trains/services but the stumbling block was Northern didn't want their Manchester crews to continue to keep 175s on their cards, as well as wanting to put 156s on Mid-Cheshire services and no longer use 158s, so ATW weren't interested.