> The main problem is that the networks are usually owned by monopoly utilities with interests in power generation. How much do you think they want to upgrade if the net result is more competition? Hell, they'd probably pay not to have to upgrade.

This... is exactly what is happening :-). Utilities and network operators are trying out ways to give incentives to customers who can respond with negative load (i.e. turning the power down). It makes economic sense because the alternative is increasing production at immense speeds, which is quite costly and inefficient (often using diesel generators, and a lot of them). Aluminium smelters like the examples you give (and quite a few other businesses) are doing precisely this.

Also note that an aluminium smelter works by electrolysis, not by pyrolysis (which is what GP seems to suggest?), which is why they use so much electricity and why they can in fact turn it on and off at a whim.