I think the argument is a bit different. There are two issues--more hub capacity and more capacity for all traffic in London and SE region.
If LHR NW at the price and with the consequences is acceptable in political terms, all well and good, both boxes ticked.
If not, then to the chagrin of some frequent posters here, we will have to give up on the expanded hub capacity but that still leaves the question of total capacity and there LGW R2 very much comes into the picture. If the economy performs as hoped/expected, we are going to be desperately short of point to point capacity in London and SE by the middle of the next decade. OK £10bn is still hugely expensive but to say 'it's Heathrow or bust' is misguided.