You're comparing flying direct vs flying through Edinburgh. What about people flying from places that dont have a direct flight to the US?
Compare flying through Edinburgh and using pre-clearance vs flying through Amsterdam or Paris.
Okay.
1. Fly from Europe Spoke site to Europe Hub site with preclearance for USA. No immigration or customs clearance on arrival at EU Hub, just preclearance.
2. Fly from Europe Spoke to EDI spoke with preclearance for USA. Immigration & customs clearance on arrival at EDI as we're no longer "free movement", never mind Schengen, then preclearance.
3. Fly from Europe Spoke site to USA with no preclearance.
If you can't fly direct, 3. is by far the quickest with only a risk of delay on landing. 1. is next best as you have only a small risk of delay at preclearance and even if you do, there's likely to be a later flight [as it's a hub airport] that you could be transferred to. Option 2. is the worst. You risk delay at UK immigration AND customs as you will
have to be landed before going through pre-clearance and then another delay risk at US preclearance which could prevent you getting on your once-daily EDI-spoke flight to the US hub airport.
I appreciate that there are more than one flights to the USA, but if you are connecting out of DC, say, a later flight to EWR
may be an option but to other destinations served like Chicago, Orlando or Atlanta are getting towards the chocolate fireguard category.
I am afraid to say that until EDI actually becomes a hub airport for [multiple] US based airlines, I'm fimrly of the mind that preclearance will remain largely irrelevant.