I remember that I forecast on this Forum in 2008 that it could be 5 years before traffic levels recovered to 2007 levels. Yet things are as uncertain now in that regard as they were then with no indication of that happening in the short term, and there is even the possibility (probability?) that growth will be extremely modest over the coming few years.
The rapid growth era is over in my view with oil prices likely to remain at least on the high side of average indefinitely and I simply cannot see the sort of investment that a Thames Estuary airport would require being forthcoming. The planning process would become bogged down in legal challenges from environmentalists and others that would go on for years.
Apart from all that if the new airport is proposed as an alternative to LHR, which would presumably close, then it is in completely the wrong place; no matter how much new transport infrastructure is put in place.