Obviously from Liverpool (or Leeds) centre to London centre the much-improved train is a no-brainer. However, it is always good to have choice and with congested roads and the railways near capacity, an air alternative is desirable. A high proportion of pax are not centre to centre.
Regretably, thin domestic routes to/from London or the Thames Valley
may only be viable if they can combine commuter and feeder functions, which means thay would have to be to/from LHR, perhaps code-sharing with BA-BD and/or VS. Without LHR expansion it ain't going to happen.
It is being suggested in some quarters that south-east airport capacity issues could be partly addressed by reintroducing civil aviation at NHT, with a high speed link to LHR, and a 17-minute Chiltern line train journey to London from a NHT airport station. That being the case, thin domestic routes operating to/from NHT could combine commuter and feeder functions and may become viable. As always, carriers would have to see business potential.
With a higher proportion of "carry-on-baggage-only" pax than on overseas flights, there could be room for belly cargo on these thin domestic routes, providing a little relief to the roads (it all helps).
Obviously, the toxic issue of APD would have to be addressed, so don't don't hold your breath. However, it's still good to be a little contrarian, someone has to think outside of the box!