IMU/INS is such a huge enhancement to aviation safety, in terms of Situational Awareness... without it and GPS, EGPWS would probably never even be possible.
The enhancement to the situational awareness is so damn significant, especially with the kind of accuracy that modern IMU systems provide nowadays, with Ring Laser Gyros.
In drones, we use MEMS gyros, not for positioning, but for Attitude Determination, which are super cheap (just google MPU6050 gyro). The reason for their low cost is mass production, since they're widely used in phones/some game controllers.