Scotland is more "local", it needs an airport for both Edinburgh and Glasgow and so the market fragments badly and the comparison is lost.
While I agree that what Scotland needs is a single, central belt airport, I dont think it would make much difference here. The main market for this flight is inbound Chinese tourists, many of which tend to book as part of a package tour. What airport they fly in to is irrelevant - as they'll be boarding a pre-booked bus and will be whisked off to the first of their many destinations - probably Edinburgh to start anyway, then on to other destinations around Scotland.
Any business travel, or Scottish tourists heading to Beijing will just be a happy extra, but certainly not the main market for this flight. As such, I don't think a single central airport would make much difference in this case.
As for the United / Delta examples - i'm not convinced. Look at Manchester, with its much larger catchment, yet it's still narrowbody central to the US. With the exception of the United flight which upgraded this year, it's all 757s. These aircraft are perfect for operating flights to the UK, freeing up widebodies for flights further in to Europe.