I strongly support Man Airport having services from across the NW. A shuttle wouldn’t even cover Victoria or Oxford Road, and I imagine demand for airport services is very well distributed. It’s the only major airport in the region, especially for long haul, so it needs to serve wider than just Greater Manchester.
The airport service not being a shuttle terminating at Piccadilly also avoids such trains having to turn right across the eastbound Castleford Corridor flow in the terminal throat area. Keeping as far left as possible reduces conflict.