In fact GPS is amazingly accurate when used diferentially to calculate attitude down to sub mm accuracy
That's not actually true in most circumstances. There have been experiments with highly specialised GPS receivers tracking L2 Phase for aircraft attitude purposes that worked well, however general GPS is simply not good enough.
Modern drones have experimented with thermal horizon detectors which work quite well in flat terrain, but have problems in hilly terrain.
The current alternative to INS is a combination of GPS and 3D accelerometers / rotation sensors that provide quite accurate attitude and location information. They do not meet full INS accuracy but at under $100 a sensor unit they are extremely good value.