I don't recall any 'press releases' re the reasons for the American withdrawal from GLA nor the United withdrawal from GLA.
'Rumours' at the time had AAL withdrawing due to them getting (supposed) better yields on a South American route with the 767-300 used on the GLA flight, the GLA-IAD United route was a weird one to start with.
NWA consolidated most of their European ops at AMS and moved the a/c to the Trans-Pacific routes, fact.
GLA still handles more Transatlantic pax year round than any other Scottish airport, but moreso in the winter when only COA operate from EDI and GLA.
As for Business Class pax, I'm told at times there are NO seats available from GLA on the EK DXB flights.