Personally not sure what United are gaining from the limited service to Edinburgh to be honest.
The
four daily inbound United flights full of American tourists and business travellers? Or the
three daily Delta ones?
Nothing wrong with seasonality, ask Jet2. And both Delta and United offer year round service to JFK and EWR.
Interesting point about the ATL frequency though, what's driving the issue? The A330 seems a good sized aircraft and with the DL JV too, so why are VS undermining themselves?