DS,
Not an unheard of volume though, the Rotterdam extension is about 7 square miles and directly onto the North Sea.
True, but still an extension in pieces (Maasvlakte 1, then 2), rather than a completely new venture. A port does tend to benefit slightly from being next to the sea
Ultimately this is a political decision, the rest of it is just engineering and fairly well understood engineering at that.
Yes, whatever they do, there will be politics, but it has to make commercial sense too, not just in terms of can it be built to give shareholders an ROI, but also how could such an airport be built by sacrificing at least one other going concern, and still satisfy (a) the shareholders of all and (b) competition authorities.
I don't doubt that an estuary airport is technically feasible, but we don't have much precedent of doing this in the UK. History also shows that even if a plan is developed for a new superhub, the older, closer (to the city centre) airport remains open, as in SEL, TPE etc, more often than it does not.