There will probably be some really interesting network effects with this. Since a lot of the other PoW coins that are ETH-hardware compatiable have low volume, I think we're going to very rapidly see the profitably of mining these go way down (to the point of going negative in some cases) as all these extra miners suddenly start mining these coins, but this takes time. So in other words, I would expect the real global energy usage reduction to happen weeks or months after the merge since it will take a while for all the altcoins to get overmined.