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The energy consumption is the only security mechanism Bitcoin has. It's impossible to drive it down.

PoS would be a completely different fork, and it's not clear how secure/censorship-proof it would be.

Any protocol change is, by definition, a fork. If everyone says the new PoS Bitcoin is the real Bitcoin, then it is so.

But what would be incentives to using a PoS Bitcoin fork, as opposed to starting over with a fresh altcoin?

You keep the existing network effects which are massive.