In terms of LBA-LHR, one of the key points to emerge from the committee sessions was commercial reality. It's all very well LHR asserting that a certain number of regional routes could be served, but the airlines have made the (IMO obvious) point that unless a given route is commercially viable, it won't be served.
Think the point made by LHR management is that with expansion, the supply-demand situation with slots changes and the secondary slot market disappears. With slots freely available the biggest obstacle to smaller operators on thinner routes is removed.
The recently announced discount on airport charges for domestic routes will help, that leaves the second main obstacle, double APD.
Obviously commercial viability is the determining factor and it would be up to pax on these routes to use them or lose them. That is why LHR management states that regional routes
COULD be served, not
WILL be served.
The current LBA route is largely a glorified slot sitter (as are some of the rotations to MAN). One of the daily LBA rotations has already been shelved and I think you are correct to say that the current one will likely go as well - sooner rather than later. However, if a third runway emerges (at great cost and with high charges) the glut of slots means the need to "slot sit" has vanished. This might make it as likely that the route goes in a third runway scenario as otherwise.
Not convinced that it is a slot sitter: why open a new route just to do slot-sitting, wouldn't it be better to increase frequencies on existing routes for this purpose? It would save the expense involved in opening and running a new station.
There is always BE, but they have been as vocal as any in explaining the problems they face with the current charges. Unless something new emerges (e.g. Subsidy) I think we can count them out.
Would imagine that BE is testing the market to ascertain if Heathrow operations are viable in the long term, and if so, get its foot in the door ahead of the competition.
It's a shame that EU rules oblige BE (and VS before it) to duplicate what already exists on two routes and leave some remedy slots unused. Without this nonsense there may have been the opportunity to try out some unserved routes.