The earth-to-earth idea is nice, and whilst it would be an improvement, I expect the travelling time to be the biggest obstacle. You have to gather the people, check them in and clear them during internation flights. Then you have to load everybody on a boat, ship them to the platform X miles into sea, unload them, take them up the big tower, letting them board and settle and depending on how predicatable this is, wa…
At the moment there's lots of time wasted walking to some gate at the far end of a massive airport. There's only one company here and one or two flights that go straight up and don't need to taxi. So the travel time in the boat won't be much more than the walk from the terminal to the gate.
That's not saying there is no market for something like this, but for anything but the longest flights it's going to be a very specific kind of customer.