Aye, but very very few people are using this service for journeys anywhere near that long - most of the passengers are on journeys of up to two hours.
At least on a service from London to Glasgow (a comparable time) you know that a number of passengers are going end to end, but really the Liverpool - Norwich service is a dozen "local" services tagged on to each other.
I am not sure that is actually completely the case.
True, the service is used by lots of people travelling short local journeys like, Liverpool-Manchester, Manchester-Sheffield, Sheffield-Nottingham, Nottingham-Grantham etc.
However, although there are very few people who travel the whole journey from Norwich to Liverpool, there are very large numbers of passengers who travel on this service for fairly long journeys of 2 hours or considerably more.
Some of the journeys (and their returns) very popular on this route are:
Norwich to both Nottingham and Sheffield
Nottingham to Manchester and Liverpool
Peterborough to Sheffield, Manchester and Liverpool
Even Peterborough to Norwich and Liverpool to Sheffield are not short journeys and these are very well used.
The longer journey opportunities that this route provides also from stations not listed above like Warrington, Chesterfield, Alfreton, Grantham, Ely are also important to many people.
This is a very important route for passengers wanting to make longer journeys without the inconvenience of having to change trains. Lots of people do not want to change trains and would often rather put up with slightly longer journey times.
It is much more than a series of local routes joined together.
There would be far more people willing to use this service if it wasn't so overcrowded. I agree that perhaps sometimes there are not as many longer distance travellers as you would expect on this route but if you have had to endure a 4 or 5 hour journey in a grossly overcrowded 2 car 158 would you use it again?
With longer trains and more space to spread out and move around the train I am sure there would be a big increase in the number of longer journeys made on this service and then, yes, perhaps even a significant number of passengers travelling the whole route.