The way I think about it is that commercial flights spend the vast majority of the time at "cruising speed", where the engines are most efficient on a power/fuel basis. The flight already dissipates 100% of engine power output into pushing air out of the way, so what altitude does is determine what airspeed that happens at. At lower altitudes, there's more air molecules displaced per foot of travel, so the airspeed is lower.

So airplanes fly up at 30k feet because it's faster at the same power output, which also happens to be more fuel efficient, but it's not like the plane runs the engines any harder if they cruise at a lower altitude.

These ion thrusters have worse thrust/watt than an electric propellor drive, and far worse than the jet engines they use.

So there's not really any reason to use ion thrusters at cruising altitude, unless the relative silence is worth paying a lot more per-ticket.

From the Ars article (for others who were curious about this):

> Measurements showed the thrusters collectively generated five newtons for each kilowatt of power, which is actually similar to the output of jet engines. But because of many inefficiencies in the system, the overall efficiency was only about 2.5 percent—well below that of conventional aircraft.

Even then, I'd imagine that a significant portion of the noise in an airliner is wind noise rather than engine noise. Your plane is only going to get "less noisy", not "silent". Good noise canceling earphones are a far better solution (I have some specifically for plane flights, they're awesome!)

Because you would need to restart the engines to land, and that is when they are at a higher risk of failure. You could maybe throttle them right back but keep them ticking over.