I agree on everything you're saying, and understand perfectly why one wouldn't want the other to get a new runway.
However, it does seem a little bit like LGW are saying "I want one, but not only that, I don't want them to have one"...I suppose having their cake and eating it?!
If, for example, Davis Commission/Government were to say "OK, you can have a new runway IF you can fund it" and that includes STN/LTN as well, then we would soon be in the position where only LHR gets a new runway because it is the only one that can fund one in terms of private investment?
That way, Government can claim it's tried to be open for everyone and let market forces decide, no bias anywhere!