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.

Ether is also scarce. You can't spend one Ether multiple times for staking.

Is ether really scarce? There is no limit to the amount of ETH unlike bitcoin, so scarcity doesn't seem to be built in in the currency as with BTC.

yes, ether is scarce at any given point.

The part you got wrong is that you think they care about decentralization. Very few people in the crypto spare actually care about it. They say they do, but that's only a part of the sales pitch for their own crypto that they pre-mined.

the BTC dev community did (and still do) a lot of bad things, but the one thing they did right is put their foot down on decentralization even if it meant a bad user experience. If it becomes centralized, then it's nothing more than the most inefficient SQL database and there would be no point in using it to being with.