I'm struggling to see how a route that's not viable (i.e. not enough demand to operate at a profit) becomes viable simply by building an additional runway at one end or another end of the route.
Because the airport is now serving the UK rather than the south east.
As for the use of a 3rd runway stopping the MAN bus service, dream on.
Your still talking about LHR today rather than with the proposed third runway. The domestic flights are cancelled due to capacity issues.