To have 6 feet between passengers, you'd have to build space in both laterally (as you said) and longitudinally, so that means removing half the seats in the aircraft. They've modeled the revenue per flight with these kinds of arrangements and they are a money-losing endeavor. Airplanes are just too high cost of machines for carting around a bunch of empty space. Not only that, but the air re-circulation in many aircraft is more beneficial to spreading disease than it is preventing it, so that will have to be reconsidered.