Whilst EK certainly seem to have found a market, I'm not sure I agree that the size of that market exceeds that for travel to/from the US. Undoubtedly outbound pax play a big part, but I think in summer a very significant proportion of pax on flights between the US and Scotland are inbound. Obviously there are less in winter, but I suspect more direct flights would themselves serve to attract more inbound business.
Even with the launch of the EDI-EWR flight, the CAA stats appear to show that GLA-EWR is still experiencing loads of 85-95% for the winter so far. I accept of course that this does not automatically equate to profitability, but it nevertheless looks promising, and I feel there is room for another US airline to operate at least 5x weekly from GLA to it's US hub. Prior to 9/11 AC used to operate 5x weekly GLA-YYZ in addition to CO's daily EWR service, so two winter transatlantic services have operated before and I personally believe they could operate again. BAA have apparently been trying to get AA or US to go year-round, undoubtedly with the help of some sweeteners from the Scottish Exec!
