There's no point in looking to what worked in the 1970s to plan for 21st century airport accessibility. The only way this would work would be like this :
Viking Sells Series 400 Twin Otters to Scotland | Viking Air
A taxpayer funded and PSO operated and subsidised operator to fly from regional airports into an expanded LHR to make sure the wealth and
opportunity are a little more spread around. There is no way in Hell that it's a commercial proposition, but there's a good argument for trying it as a PSO, assuming LHR gets a third runway.