I'm not sure anyone would have envisaged the huge growth in air travel that we take for granted now.
do you mean the growth to where we are now, or continued exponential growth ad infinitum? This seems to be the basis on which the FBI concept is propagated, and this is highly unlikely. Yes, we have reasons why people will continue to fly, and there will be the growth areas in Asia and Latin America, but markets like the USA, on which much of LHR's dominance lies, are pretty mature.
I can accept a case for a 3rd short runway, but not one for a new airport on the scale being suggested in the Thames, or at least not without depending on the huge volumes of "virtuous circle" transfer passengers mentioned in the model. Somehow they've forgotten that the transfer pax are not as valuable as o&d, so an airport relying on ever-larger numbers of them is a financial house on the sand as well as a technical one.
The third and/or fourth runway for LHR is all very well to cope with increased air traffic but significant expenditure is required on the infrastructure surrounding LHR and beyond.
Except that there is infrastructure already in place around LHR, whereas a new airport means starting from scratch, so a
far bigger problem.
I don't understand how proposing either of those options constitutes "acceptance" that the Thames hub scenario is "nonsense".
No, when they are proposing that LHR merely "tones itself down" instead of their original plan to close it, it demonstrates that the arguments for the fantasy airport are getting weaker. Either way - tone down or close, both scenarios present huge problems for the new airport. Obviously, leaving LHR open as it is cannot happen either.
Would imagine that it would be employees/staff cycling on to the airport rather than pax.
Yes, and this is the problem. For any other type of development, you can say "we will generate x,000 vehicle movements per day, so we will mitigate this by adding 0.5x worth of road improvements, and we will also improve the local pedestrian / cycle / public transport system to the point that there's a reduction in vehicles of another 0.5x - thus there's no net change in traffic.
You can't do this so easily for an airport, as very few passengers will cycle, but you can provide a comprehensive cycling network around your site and you can provide money for a network in the nearby areas, thus reducing traffic and air pollution.
These are the biggest challenges LHR faces if it wants to expand - not the pollution from the aircraft, which is largely dispersed at height, but ground level air pollution and traffic congestion caused by road vehicles.