Agreed, never bought into the idea a new route would be launched just to slot-sit if it was not profitable as well. Also not suggesting that LBA and INV are profitable slot-sitters!
The INV-LHR route has the potential to become a route served by more than one daily frequency with BA if demand dictates. I personally don't believe BA are launching INV to slot sit. However equally BA won't continue to operate the route if it is not profitable. I guess you could say it is going to be a case of use it or loose it.
I know that BA have made the price of the fares as competitive as they possibly with some cheaper than EZY by the looks of thing. Now all they need to do is work with HIAL on marketing and advertising and I am sure the route will be success