The rotor spins in a vacuum to reduce standby friction losses, and it requires power to maintain that vacuum. The bearings also need active cooling. Without these support systems you won't have a catastrophic failure, but you'll be damaging your equipment and reducing its lifespan.

As for using the flywheel(s) themselves as the source of backup power, that was our original design and definitely feasible at a conceptual level, but there's a lot of engineering in getting that to work properly while maintaining grid code compliance. You need your grid-tie inverter (which also provides the 60 Hz AC used by the support systems) to disconnect from the grid and transition to island mode /without interruption/ very, very fast (since utilities have standards on how fast generators need to disconnect during a system fault) and basically it required us to write our own firmware for the VFDs we were using which in turn invalidated their safety certifications. So definitely a solvable problem but we just didn't get there.