I did a day return from Newcastle to London by train yesterday, left Newcastle at 0704 arrived Kings Cross at 0940 left at 1500 back into Newcastle at 1752. Simple easy and surprisingly busy.
The train is great for city centre to city centre travel, provided that one of the city centres is London. For most people who do not live in a city centre, a journey by air may be preferable,
and quite often cheaper and, subject to proximity to an airport, often quicker.
One doesn't have to live far from London for it to be a 2 hour journey plus, with several changes, just to get up to London, before even starting a 2 hour train journey to Leeds/Manchester or 3 hour train journey to Newcastle. Also one needs to consider that travel to/from Heathrow by train involves going via London and using different London stations plus the tube, while carrying luggage.
Domestic routes to/from LHR offer a one stop connection to the world, so LHR-LBA is not surprising especially as no one else is on it, and there maybe scope for code sharing. Would have expected another unserved domestic route rather than LHR-AMS, LPL perhaps (apart from the Liverpool conurbation it also has an additional populated catchment area in Flintshire/Denbighshire). LHR-LPL on U2's list if it ever starts Heathrow operations. Alternatively NQY, a route the old BE did well on, four times a day. Would it work without the PSO arrangements?
Also bear in mind that with weekend engineering works, rail travel is often a 5 days/week option. As for road travel, endless roadworks, death-trap smart motorways, heavy congestion, and speed cameras make this horridly slow and unpleasant.
It's good to have as much transport choice as possible and if a route doesn't pay, it will be pulled.