It's already been proven that you can secure a network without wasting massive amounts of electricity

has it though?

I'm not sure which of the many consensus protocols you may have in mind, but proof of stake is the most popular alternative.

A 51% attack for PoW requires that someone can afford to power 51% of the network. That's expensive. Once the attack is detected, the community can fork away from the moment of the attack.

A 51% attack for PoS requires that someone owns 51% of the tokens. At that point, the attack costs nothing. If the attack is detected and the community forks, the attacker still has enough tokens to attack again on the new network - at no cost.

All of this stuff is brand new. Saying that any of it is "proven" is pretty bold.