I don't know the ratio of passengers who flew onwards from FRA though LH did report that in the first six months of the route BRS-emanating passengers used all their worldwide connections from FRA.
I know the route was very valuable to many in the West Country who used it for onward travel.
The route lasted for 12 months until the end of April 2009 and carried around 98,000 passengers in that time. LH wouldn't have stopped flying the route on a whim so presumably there were better pickings for its aircraft (actually Eurowings of course) elsewhere.
When it ceased to operate the LH UK manager gave the impression it was being suspended rather than axed because of the recession that was beginning to take a hold.
She said then that she saw no chance of it returning in 2010 which led some to believe it might in 2011 if economic conditions had improved. It appears it's not coming back imminently so one has to now wonder whether it will come back at all.
LH has routes to FRA from LHR and BHX and might consider that they can serve West Country travellers from there.
As for low cost airlines, easyJet doesn't fly to FRA and Ryanair uses Hahn. Flybe does fly to FRA from MAN, BHX, GLA and SOU so conceivably would be the best bet, but then again they might consider that BHX and SOU serve the needs of West Country passengers, especially as Flybe would need to sort out slots at FRA for a BRS route.
I would much prefer LH to return rather than a point-to-point airline as it would be much more useful to me.