Some of the posts here seem to assume this is being put forward as an alternative to Heathrow, & write as if they expect Heathrow to close if this airport is built.

It’s being put forward as an alternative to expanding Heathrow. In the unlikely event that it’s built, Heathrow would lose traffic, at least in the short term, but I can’t imagine it closing – and I would think the older terminals, & the tangled mess around them, more likely to close (or be demolished & rebuilt in a more efficient layout) than T5.

I think it’d be more of a threat to the existence of the minor London airports than Heathrow. There’s too much invested in Heathrow, & it’s too convenient for too many people, to close it. As for the jobs, etc – well, W. London & the Thames Valley has a lot of scope to soak up any slack resulting from a diminution of traffic through Heathrow (normally – not at the moment, but the current recession should be over before any impact would be felt from a new airport), & look at the proportion of the population which is transient, & could simply move away. The Thames estuary is comparatively depressed, with much higher unemployment & much less pressure on land. Moving sources of employment from here to there would probably produce a net economic benefit. BTW, that’s one of the arguments put forward in favour of the proposal, exactly as it was last time round.

I see the flaws in the idea (& they’ve all been rehearsed before, when the original Foulness – or Maplin Sands – airport was mooted – though I note this proposal addresses some of the drawbacks of that plan), & don’t believe it’ll ever be built, because of them, but there’s no need to invent spurious arguments against it.