However Stevenage *is* on the East Coast Mainline - so much closer than Luton Airport - try harder.
Don't forget that driving to a suitable station is an option too. Many people in the Home Counties wanting to go to Manchester do so by driving to MKC and parking up there, for instance, and the idea that someone in the Home Counties might, rather than going via London, drive to Stevenage and take Lumo (say) is not at all far fetched - people *do* do that. Airports aren't the only transport hubs you can drive to.
If you consider driving to the airport fair game, so is driving to a suitable station when making a comparison. If you don't consider driving to a suitable station fair game, then public transport should be used to get to the airport, and that rather changes things.
Same is true of the train though, unless your destination is central Edinburgh in the vicinity of Waverley station. I don't think you can argue that one has the advantage over the other.
This might work for Glasgow, but Edinburgh is quite a small and an anti-car city and as such is laid out in a very traditional "city centre centric" manner, with almost everything a visitor is likely to want to go to in the city centre, and (unless being picked up or taking a taxi) that being a better way to access most of the suburbs by bus too.
Thus unless you're visiting someone living in Corstorphine, Ratho or surrounds, or going for a work trip to RBS Gogarburn, the airport, despite being quite close in to the city centre compared with most cities, isn't likely to be a useful destination.
That may flip round (see Luton) if you're travelling *from* Edinburgh of course - the general pattern for business trips (say) is suburb to city centre or suburb to business park.
I know people who have flown Luton to both Edinburgh and Glasgow regularly - their view is 30-40 minutes before the flight leaves providing you only have hand luggage is eminently reasonable.
It's probably doable if everything goes perfectly, but does carry a significant risk of missing the flight. Interestingly with BA at Heathrow you'd be turned away - if you don't scan your boarding card at security 35 minutes before scheduled departure, they offload you. I'd stick with walking through the terminal door 45 minutes before, which as the car park is a short walk from the terminal means handbrake on one hour before, is the minimum sensible to ensure you're going to make the flight.
But even so it still isn't two and a half hours, you've missed way too much out of that.
And with the DART now open, parking at Luton Airport Parkway station and using that will be ~15 mins from parking - which offsets the time driving up the hill to the airport nicely, especially if you're arriving in Luton from J10 of the M1.
That's possible at the moment (and I've done it when the buses operated), but I suspect once people start getting wind of that possibility the car park will become rail users only with a need to show proof of rail travel, as it'll get so full rail users won't get a space. Of course that, in practice, means pulling on your handbrake maybe 1h10 before departure, as there is a short walk from the DART terminal to the actual terminal and you need to take account of the fact that you may just miss one. If you are wanting to cut things super-fine just to prove a point, the short stay multi storey is the only sensible choice.
The actual approach road hasn't been seriously congested since it was dualled and the airport end improved, by the way, so you wouldn't save much there.
But none of this tweaking around the edges detracts from the point I'm making, which is: yes, it's quicker to fly, but it's not *that* much quicker, and if you take into account stuff like potentially being able to work on the train (far less so with the bitty time available in each place when flying) it can be the case that train is preferable despite taking a bit longer.