It would be interesting to know what the cost to the airport would be for pre clearance. It sounds as if the money might be better spent on infrastructure.
I strongly doubt it would attract new airlines or routes. The truth is that airlines are not hugely interested in how long it takes passengers to clear immigration, particularly if the pre clearance is reflected in the ticket price. If an airline sees potential financial benefit from a route, it willl try it, irrespective of facilities. Where it makes a difference is if an airport has a based airline which can funnel passengers between e.g. European and US flights. Edinburgh has no prospect of this (only LHR or LGW do in UK).
Given that such a large percentage of EDI passengers have US passports, it is of limited benefit. Would be better at LGW or MAN which have larger passenger numbers to US, paticulary off season, with a much higher percentage of non US passport holders. But they have traditionally been cool on the pre clearance idea. There must be a good reason for that which Mr.Dewar is unaware of.