Well at the moment there are only 10 services per hour from Manchester Pic to Manchester Airport. Back in the days when a reliable timetable operated (the December 2017 timetable was the best one the region has ever seen in my opinion) there were... 11
Actually there are only 9tph from Piccadilly to the Airport (8tph in the peaks) versus 10tph (9tph in the peaks) before May 2018. Nevertheless your point is well made.
A significant difference is that, of the current 9tph, 8tph (3 TPE, 4 Northern, 1 TfW) are from the Piccadilly through platforms, with only 1tph (TPE) from the main shed. Previously only 5tph (1 TPE, 3 Northern, 1 ATW) were from the through platforms, with the other 5tph (3 TPE, 2 Northern) from the main shed.
Clearly the congestion in the Castlefield corridor has adversely impacted performance, particularly the rerouting of the TPE North services via the Ordsall Chord. And, although there has been a net reduction in Northern services, the new ones are from Liverpool via the CLC line and Preston via Golborne, whereas the replaced services were from Southport via Bolton and from Piccadilly (x2). The longer routes have also increased vulnerability to imported delays.
Addition of a service from the Calder Valley via the Ordsall Chord would clearly risk a further deterioration in performance, unless it replaced one of the existing services through Piccadilly.