While you certainly have a valid point the more savy customers may well travel to save a few pounds many families holidaying still want to travel from their LOCAL airport.
The road trip to EDI is not a pleasant experience as its a single carriageway road for much of the way, clogged with trucks if you time it wrong. The train doesn't go to the airport so that's not a great alternative.
Business travellers won't move to EDI en masse either as they want to travel to where their work is, why spend 2 hours travelling from EDI to a meeting in the North East when your flight from Europe was only 90 minutes in the first place?
A lower or zero rate Scottish APD is certainly a great concern to the airport but not necessarily the doomsday scenario painted here.