Indeed, you are mostly right on these issues - probably much to do with the fact that you know exactly how the people who plan this stuff think and probably speak to them on a regular basis.
That seems a very UK- and London-centric view. London is by far Zurich airport‘s largest destination, and since Zurich Airport is the one for the East of Switzerland, Basel would perfectly serve a a railhead for exactly the same region. For a typical Swiss, it makes zero difference whether they take a train to Zurich airport, or to Basel SBB. Obviously, you would need to go for the Swiss customers rather than fill the train with Londoners, which seems to be E* plan.
BTW, do really that many people travel from London to Frankfurt? Because I doubt the German side is going to fill these trains if they don’t serve Cologne - and thus the whole Rhein-Ruhr region - too. I don’t really know, but apart from Bankers, Frankfurt strikes me as one of the least attractive destinations in Germany, and the attractive places that way are quite a bit away from it.