I think you're making the improper assumption that the drives would need to be left on during this entire time. I don't believe they would.

I don't know if the bits eventually lose their magnetism over time, so if they do, you may need to spin up the drives every so often and copy to and from drives to make sure the data is still "fresh", but I seriously doubt they'd need to be left on and spinning for the entire 50 year span.