LGS, you make a valid point about areas of affluence now but we need to be considering the future and that is why Stansted is extremely viable if Heathrow really is out of the running. The East (of London) is getting ever richer, and has the biggest pockets of land available for development over the next 10-20 years - look up Dagenham Dock and Barking Riverside for example, with rail upgrades and DLR extensions very much on the table.
All Stansted needs is upgraded track into London, and a second rail link into the Liverpool St-Norwich line can be easily constructed by bringing back to use the Braintree - Bishop Stortford line to the south which would improve access to East Anglia via Colchester, Ipswich and Shenfield. Connections north via Cambridge could also be done.
Improved road access to the west to better link in the Midlands and beyond would also be doable.
But ultimately Heathrow is what should be expanded in a sane non-politicised world.