Solution: giant tunnel to underground downtown terminal.
Planes land outside the city, taxi to the downtown terminal.
Planes land outside the city, taxi to the downtown terminal.
If you're suggesting this hypothetical giant tunnel would actually have the planes taxiing all the way to a whole airport underneath a city... I don't see that solving more problems than it would create.
It would create problems, but the problems would be for the designers and maintainers of the infrastructure, not for the passengers.
Airplanes are not fundamentally that much bigger than trains, which routinely go under the city.