It is an interesting debate and I cannot pretend to know about yields and what makes money for any operator.
I have used BD on many LHR-GLA-LHR runs...and the loads vary quite considerably in my opinion. Indeed thefirst north bound flight on a Saturday Morning not that long ago had 17 pax but yes I have been on fully laden A319's and A320's
I am old enough to just about remember the DC9 service all these years ago that had good loads and possibly at prices similar to days cheapest fare. But I still assume that business people are willing to pay the fares asked for short bookings. Perhaps its actually the service on board that BD is now lacking. Apart from the seat allocation they are effectively like all other low cost operators - buy on board etc.
But yes I would be sad to see them go and wonder just what would happen to the prices on BA when they have sole rights on that service to LHR. I also feel that once again the GLA users would suffer with the loss of a one stop - baggage checked through to destination use of Star Alliance - and that is a shame. Its hard enough i am sure for many from GLA having to fly south, just to fly North again for trips to say North America and having to route even fuirther into europe like FRA (if Gla gets such a direct route) can be also frustrating.
Needless to say GLA-FRA etc has been tried on a number of occassions and even if it was re-invented with smaller ac etc - I just wonder how long that would last to.
Nivsy