I think BA and Virgin would still have a smaller operation at Gatwick even if Heathrow had the space. The more leisure/holiday routes go from Gatwick, and major cities/hubs from Heathrow, ie. Heathrow is places like Singapore, New York, Delhi etc. whilst Gatwick is Orlando, Montego Bay, Male (Maldives). It's a clear split of different markets.
To what benefit? It's a clear market split because hub and spoke is prioritised at LHR and beach leisure is left at Gatwick. There's no business reason to keep it there if you get more space at your main base. Why duplicate staff and engineering costs when the market will move to LHR with you? What commercial benefit would there be in duplicating a split operation? None.
He's seeing potential and making Stansted as attractive as it can be for a range of airlines and destinations, clearly something that you can never see, or accept!
It's the market that says this, no one is preventing American Airlines flying STN-JFK and it's not all big bad BAA's fault they don't.