The trouble is that the factory costs something too. If you're only running half the time, now you need to spend twice as much on factory to make the same amount of stuff each month.

So if we build a wind/solar grid without enough storage, this sort of demand management isn't a free lunch. It's just that instead of the power company paying the cost of intermittency, it'd be externalizing that cost to everybody else, who pay in higher capital costs.

(For aluminum in particular you have another problem: if you shut down more than about six hours, the molten aluminum freezes and you spend a lot of money on repairs.)