Comment by rexskimmer

> And bearings/motors are such mature tech that they can last decades, even a century without replacement.

This is not true at all for bearings in large industrial systems under constant load. As the mass scales up, the systems become exponentially more sensitive to vibrations from wear of the mechanical parts. Even magnetic bearings max out at about 20 years.

Flywheel systems are more complicated and expensive than simple chemical batteries. Cost also scales much more than batteries as you increase the energy storage. They are also less efficient over time due to friction even with magnetic bearings.