The problem is that Luton Airport has a national infrastructure role, and is not just a local airport servicing local demand. Until a third runway is built at Heathrow - if it ever is - there will be pressure on the other London-area airports to add capacity.
It is a bit fanciful to hint that Luton would have a role in relieving pressure on Heathrow. Heathrow has been full for years while Luton has always had capacity. While airlines have been willing to pay millions for Heathrow slots they haven't exactly been falling over themselves for a touch of the Luton experience because they want more of London or is it that they really want the Heathrow connections?