It does seem that the whole thing may be an exercise in lambasting BAA to create a scapegoat (and secure discounts for remaining routes) before they eventually can GLA, MAN and most other LHR domestics, even though the routes are loss making for many other reasons.
In terms of BD potentially not flying GLA-LHR any more, if people are going to N America they can use CO (sorry UA!) to EWR or US to PHL.
The last time there was a regular direct service from GLA to FRA was about 15 years ago when the airline business was very different, so I don't see previous performance on the route as any indication that a new service would be unsuccessful. However, until FRA's new runway opens, I don't think anything will happen.