As someone who regularly has to travel from Gatwick to LHR to connect with flights (following BA's cancellation of the JER-LHR service), a longer taxi time is but a minor inconvenience. In any case, a 7,000' runway should be able to take anything up to 777 size - even a 747 in dry conditions.
As for developing regionals, this isn't going to solve the issue of access to LHR; it's what airlines want and it's also in the national interest, economically. Speaking again from a Jersey perspective, I can tell you that the replacement of the Heathrow route by warmed over 146s to Gatwick isn't even near as desirable - even if only travelling to London - as Heathrow.