If you actually do a little reading, power draw is proportional such that each user only supplies the power required to service their own requirements. So it's scalable - as more users are added, more power is made available. Users don't power other users' services.
In the event of a power failure, one user with a UPS can provide enough power to continue their own service. So the telco doesn't have to arrange connections to the power grid or back-up batteries, saving money.
I've looked at mutiple g.fast manufacturers and most seem to offer this feature.