It could almost lead you to think that there might possibly be more merit in running 4 tph Liverpool to Newcastle via Victoria and Leeds, enabling an even timetable, rather than a) being obsessed with sending long distance trains to Manchester Airport to the detriment of all else and b) having a supposedly intercity route terminate in small to medium sized towns rather than major cities (towns which would be better and more appropriately served by lower cost connector services that can get them to more frequent and reliable longer distance services at the nearest major location)...
I think there's a lot of merit in the idea of a consistent fifteen minute frequency from Liverpool to York (the only question I'd have with your example is what to do with Middlesbrough/ Scarborough, which both ought to have a service at least as far as Leeds, in line with
@IanXC 's comment) - it'd be like the "simple" Thameslink service that used to operate, with four/hour to Brighton (rather than trying to divide those four trains per hour into various different branches), which was more reliable.
I suspect a lot of TPE customers want a through service to the Airport
That's the problem with these arguments - if you give them a blank question then a lot of people will "want" a through service to Manchester Airport - but do enough people use it on a daily basis to make an hourly (or half hourly) service from your local station worthwhile?
I have uses the direct train to the airport at all hours of the day many times and never ever seen it as little as 20 people on a train
Last time I scribbled the figures on the back of a bus ticket, the average arrival/ departure from Manchester Airport had around thirty two passengers. Not insignificant (in terms of Breich-like levels of demand!) but that's a lot of empty seats on a three coach train (185, 323) or a four coach train (319, 350), considering how stretched fleets are in the North.
We have nine trains an hour from Piccadilly to the Airport, but (because they are long distance ones) that means some awkward gaps.
From the Airport, you have three services in the first thirty minutes of the hour (xx:10, xx:19 and xx:29) and then *six* services in the second thirty minutes of the hour (xx:37, xx:40. xx:47, xx:50, xx:53 and xx:58).
From Piccadilly, you have six services in under half an hour (xx:07, xx:13, xx:17, xx:20, xx:29, xx:33) and then a seventeen minute wait until the xx:50. I can't think of another line with such a high frequency yet such a big gap in the service!
Whilst there are some key flights/ busier times of the day (when direct flights may be more required), the off-peak service is overkill and a messy timetable for anyone doing a simple journey like Piccadilly to the Airport. It also means that the "local" stations on the Airport line get a pretty poor service (compared to others in Greater Manchester) because they are at the mercy of long distance trains.
But I don't see it changing because the handful of passengers doing a once-a-year journey like Cleethorpes to Manchester Airport are more important than daily commuters wanting simple reliable services to the nearest city. That's politics...