I would tweak it and make sure Cleethorpes/Doncaster/Sheffield and Stockport retains a direct connection the airport
Right, so who do you propose loses 1tph to Manchester to accommodate that?
Having a service that reverses at Piccadilly is an operational nightmare in just about every way. Flat crossing of at least one other line in each direction (for most platform/line combinations). Twice as many paths needed for one train. Detaching and attaching required at Picc to make the Airport platforming work. You've got to have a mighty big market to justify that.
Cleethorpes, Doncaster etc. to the Airport have got to be
very marginal markets - in the thousands a year I'd have thought. Hardly a great reason for causing that level of upset.
Sheffield to the Airport is more arguable, and probably in line with Liverpool to the Airport, but it shouldn't come at the cost of ensuring that all the GM suburban stations have 2tph, roughly evenly spaced, into Manchester. Particularly when a similar level of connectivity, and journey time, can still be delivered if you have a reliable and frequent connection to the Airport.
As for Stockport, it'd have to be a pretty unusual set of circumstances to justify going to the Airport by train! The fast bus does it in 15 minutes, and most people will simply drive (although parking costs are a consideration for such a short journey).