Your point on connections works for Manchester but the forced train opton will make connecting passengers use KLM / Lufthansa / Air France and consequently see fewer jobs at Heathrow as the feed bleeds off abroad. It's much quicker to connect over KLM ex MAN / AMS than get the train to Heathrow and connect to BA or another British carrier.
Passengers prefer to use KLM/AFA etc. It is less hassle, and usually also cheaper. I'm not talking about getting the train to LHR, that would be totally stupid when the superior European options are available. I don't think the number of jobs at LHR are much of an issue in the North West considering the number 'lost' at MAN, and previously LPL by a certain LHR based carrier. Some at LHR may be totally obsessed by the place, but it means nothing to us here, so don't make out this runway is for the benefit of the whole UK - we don't need it, our needs are catered for already.