Elements of both sides are right or wrong.
There is a strong market for premium within the direct catchment of all London’s airports, certainly enough to sustain some basic routes. They all have wealthy catchments and high-value business. That is just fact and cannot be disputed. Heathrow ‘supporters’ are going to have to concede that one.
Stansted ‘supporters’ are going to have to accept many markets are both inbound and outbound, Heathrow will always have the advantage not just because of legacies and alliances but because it’s the most famous internationally. That doesn’t mean routes and airlines can’t work at STN (or LGW), but that for many of the airlines we’d be talking about overall they gain many advantages being at LHR. Sadly there aren’t too many that are able or could justify running two or more London routes, Emirates works because of the nature of the operation.
Whilst STN is considered by some inferior, there have been examples of airlines moving away particularly to LGW and finding it wasn’t the pot of gold in comparison they hoped for.