What you have to remember is for fast Liverpool-Manchester services Northern offer 156s currently and compete with TPE's 185 services and EMT's refurbished 158s. If passengers think the 319s are inferior they'll try to use the TPE and EMT services instead. I posted on here once before that when I boarded an ex-LM 150/2 deputising for a 156 on a Manchester Airport to Liverpool service one passenger got on at Piccadilly and straight back off declaring he would get the next train instead (the TPE service a few minutes later) and two other passengers sat in front of me were commenting on how bad the seating was.
I've never understood this obsession with always wanting new or refurbished trains. There will always be rolling stock which is at the beginning of its life-cycle, some which is in the middle, and some which is nearing the end.
Liverpool to Manchester is not a long journey, the design of the 319 is perfectly adequate and acceptable. I realise that some of the trains are in a worn condition, however this is a separate issue, and if the interior was restored to its original condition the ambience would be perfectly acceptable. My local service has class 317s, which are more-or-less in their original 1980s condition (older than 319s), and I find them quite comfortable, providing they are not crush loaded. I don't see or hear passengers complaining about the state of the trains.
The North West has been fortunate in securing investment in electrification - electric trains accelerate faster than diesels, require less maintenance, and are more reliable. Generally it's more cost-effective to run longer formations. Given the choice, I'd prefer the extra length of a class 319, combined with the other benefits of an EMU, over a 185.
I'm coming to conclusion this is a more imagined than real issue, I'm sure the 319s will be well filled when they arrive in the North West.