Says who. Why bother with high frequencies if you're not going to let people use them. Why have a thousands seats on a train if you're not going to let people just walk onto them for a Manchester-Airport journey. What happens when there's a delay to your inbound train and you have to get the next one?
Go down this route you might as well call it "London White Elephant"
HS2 is not about Manchester to Manchester Airport or London to Heathrow (via OOC) journeys. Why does the Ringway tail have to wag the dog of the rest of the railway network so much?
The Manchester Airport station is more of a "South Manchester Parkway" for journeys to London, effectively replacing Stockport. It's called Manchester Airport because that's where it is, that's all.
It might be convenient for those journeys, but its operational approach should not be predicated on them any more than Avanti West Coast's operational approach should be predicated on Milton Keynes commuters. (Notably, the tail often wags the dog the other way - LNR's ill-advised conquest of Liverpool passengers made life very hard for their core market of commuters for a number of months).
What about Old Oak Interchange?
Got to show what the station is partly for
"Old Oak Interchange for London Heathrow Airport"?
(note: ref the above, this would be for people travelling to Heathrow from the north; it's not going to harm Crossrail's business one bit)