It's a frankly insane piece of engineering, and that insanity is only multiplied by the fact that it works, and multiplied again by the fact that it works exceptionally well. No gimbal lock. No ball bearings.

"Actually it's really easy to precisely know your position and velocity in space, just float a special metal sphere in some fluid and touch it super gently"

I only know how exceptional it is because the video I linked above compared it's declassified specs to publicly available ring laser gyro specs. I am not a domain expert. There might be military inertial systems that beat even that nowadays. The US has been over-reliant on GPS guidance which is demonstrating it's weakness, but we used to be very very good at inertial platforms. However, that platform was so precise and accurate that improving it won't actually increase your missile accuracy that much. So maybe we have cheaper versions.

You probably know this but for trivia: Another standard ballistic missile position fix system is that they boost themselves up into space, and then take a moment to look at the stars, which is a remarkably workable system itself.

These floating balls of magic were essentially hand built, hand calibrated, with some components having upwards of 10k tests for verification. They cost several million dollars per system back in the 80s.

This is where a large portion of your tax dollars in the defense industry go: Paying very skilled americans to do very precise labor here in the US. General dynamics for example is about half the size of Pepsico, and takes a similar profit margin, but instead of overpriced water and potato products, we sometimes get the most advanced metrology money can produce.