The materials you're probably talking about are rare earth magnets used in their bearings -- the jury is kind of out on their sustainability since they're rare

The materials you're probably talking about are rare earth magnets used in their bearings -- the jury is kind of out on their sustainability since they're rare and depletable but highly likely to be recycled at the end of a turbine's life. The lifespan questions are different -- wind and constant mechanical motion put a lot of stress on turbines and I believe as some other commenters pointed out that the actual lifespans often haven't measured up to the predicted ones. Makani's capex for a kite should be pretty low; as I recall, talking to Saul like 8 years ago, the difficulty was in stable control of the tethers, not in the economics if control was achieved. But who knows where they're at now (or even exactly how good my recall is).