An interesting idea, however if this were to eventually happen I think the government would have to step in. You have to remember that the likes Glasgow, Edinburgh, Aberdeen and Inverness have high numbers of business travellers with their final destination being London, i.e. they are not interested in onward travel. If these routes were cut then it would severely affect the economies of the towns and cities North of the border. If there was a lack of direct and frequent flights to the capital then it might put off a lot of businesses from locating here.
While I understand the finite number of slots at Heathrow and that these domestic flights might not be great moneyspinners, they are vital for the economy of the country. Airlines (and the airport authorities) also have a responsibility to provide a service to the country.
Remember, Heathrow is not just one of the world'd most important 'hubs' its also the main airport nation's capital and business centre.
I guess, the other option would to start (or increase) direct flights to LCY. That would make a lot of people very happy!