[70rd]

Scarce energy does not secure PoW, the value of the mined currency does, in the same way that scarce supply does not secure the gold standard, it's the value of gold (the 'fiat' shared illusion that it's a reserve).

I can fork a PoW chain with a difficulty bomb so that the difficulty is 1000x that of mainnet for any given hashrate. Energy is still scarce, you can't double spend energy on my chain and the 'real' one. Yet my chain isn't secure, because no one will recognize it as canonical: the rewards it yields aren't worth expending effort for.

I think you're confused about what the chain is securing. It's not who owns what, it's who spends WHEN. Distributed consensus of ownership does not require PoW/PoS, these were invented to solve double spending/censorship attacks that were a problem in P2P networks.

You can't attack a chain to muddle the past, only the present. A successful attack on ETH won't change who holds ETH, it will only prevent agreeing on who receives it.