Xcode 9.0 simulators "disappeared"

In case anyone else gets here and are in my situation, I just upgraded to Xcode 9.2 and the simulators were all missing, but quitting and relaunching Xcode restored them.

In Xcode 10.3, I also lost all simulators but got them back by running

sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService