Number of destinations isn’t particularly relevant to an airport’s overall importance; it’s more about the volumes and value of throughput (i.e. passengers and cargo). Heathrow is a “high value” airport, whereas Luton and Stansted are not. Note how Heathrow only has one real ‘low cost airline’ – a relatively small Spanish airline rather than the giants of easyJet and Ryanair. The ‘London airports’ combined probably already have enough runway/terminal capacity to accommodate the demand for air traffic and relieve Heathrow, but the major world airlines congregate at Heathrow and it would take major players like BA and Virgin to switch flights to other airports to change this trend. Theoretically, you could make Heathrow a quieter airport by making it for Oneworld alliance members only (BA and partners) and then moving one airline alliance (e.g Skyteam) to Gatwick, another one (e.g. Star Alliance) to Stansted, Virgin could go to Manston (!), other non-aligned carriers to Luton, smaller players at an expanded Southend or Oxford. All linked by high-speed rail.
It’s never going to happen though as Heathrow is very much a connecting hub as well an airport for London. Connecting passengers passing through Heathrow from and to other countries are not affected by UK passenger air taxes. BA and partners want to expand at Heathrow and make the airport as efficient as possible to compete with state-of-the-art transfer hubs in Europe and the Middle East such as Amsterdam, Munich, Zurich, Frankfurt and Dubai - (Paris CDG is also major competition, but I’d struggle to call it efficient). Passengers from the rest of the UK travelling on medium and long-haul routes (particularly from the regions) are increasingly ditching the “Heathrow transfer” and going via mainland Europe or the Middle East – this harms BA and UK plc as the revenue goes to a foreign airline and a foreign hub airport. Short-haul passengers from the regions are just flying direct to their destination from their local airport with Ryanair/easyJet/Flybe e.t.c, rather than flying via a London airport.