The tail is wagging the dog, as perfectly illustrated by the fact that we have to have this thread because of how much ire any criticism of Manchester Airport's privileged status attracts on other threads.
The lack of terminating capacity in central Manchester is in many ways a red herring - more trains could run through to other destinations. The various Greater Manchester authorities own the airport and have a financial stake in promoting passenger numbers, and use their influence over land transport policy to do this, particularly at the expense of the north's other airport, most of which aren't rail-connected. So, we end up with a situation where the whole north of England network is distorted around serving a single destination, resulting in a situation where hundreds of millions are spent to bring about substantially less reliable services. In other words, everyone else (including people trabelling to and from the infinitely more important destination of central Manchester) suffers to facilitate the relatively small number of people travelling to Manchester Airport, and the financial ambitions of its owners.
The whole situation is a joke, but this post will no doubt shortly be swamped by the usual suspects for whom airport growth is a totemic symbol of Manchester's supposed superiority over every other provincal city in Britain. These same people seem not to worry that their pet megalopolis has a local transport network which is pathetic by the standards of any advanced country, or that services to the infinitely more important destination of central Manchester (somewhere tens of thousands travel to or through every day, rather than once or twice a year) are being wrecked so the airport can boast about how many direct services it has (no matter how painfully slow and unreliable they are). This airport-related penis-waving is frankly infantile.
Having said all this, if this is the situation we are in, we should at least try to ameliorate the damage caused to the rest of the network by building the western link. If direct services from Warrington and Liverpool were diverted that way, and services from Southport, Blackpool, Cumbria and Scotland were re-arranged so services alternated between going to the airport and Piccadilly first (before carrying on round the loop and back to their origin), it would reduce the congestion in central Manchester. It would also facilitate a better route for regional services from Manchester to Chester and North Wales, which could run via the Styal line to the airport then via an improved mid-Cheshire line on the other side. (Some services from this direction could continue to go via Chat Moss and Victoria, but become longer-distance services across the Pennines, as is partially planned anyway).
Spending on this link should be made conditional on funding either direct rail links, or at least useable people mover-type connections, to Liverpool, Leeds-Bradford and East Midlands, and long-distance services at Newcastle. If air transport is so vital, it's about time there waa a level playing field between competing airports, rather than the entire ground transport system being skewed in favour of one place