It would be a relative loss in revenue as well as the WG going mad. Cardiff Manchester earns a big chunk of cash, more than what cheap tickets via LNWR would bring. Especially with the journey time penalty it would bring to TfW passengers. Without finding an extra path, the LNWR arrives xx.54 and the TfW path is xx.30 northbound, southbound is better as the TfW arrives at xx.10 with the LNWR going at xx13, you couldn't actually make that any better if you tried to join a path, but it doesn't work as a connection to the two journeys as Crewe requires 10 minutes. If you tried to extend the LNWR path, it would need to leave at xx.56, and be reliable as hell or the down fast Glasgow behind it would get clattered. The next issue is the Crewe stopper, that leaves at xx.46, you will run that down at Wilmslow.