Think of Ethereum instead as the tamper-proof store for the audit log of the history of all smart contracts in the world. While in the early days you could just transact on that, it doesn't scale if everyone's trying to "write to disk" for every single transaction all at once. So the article, and the companion piece by the same author at https://every.to/almanack/defriday-2-polygon-s-surge-ethereu... describe ways to have other, more lightweight chains that periodically flush their changes to the Ethereum chain. It's elegant, and as with all things cryptocurrency, just requires people to believe that running validator and miner nodes is worth it.

What guarantees there are for what happens off-chain? Because if that is garbage, the Ethereum will store tamper-proof garbage.

The thing off-chain is itself a blockchain, with miners incentivized to work in good faith. I imagine, though haven’t verified, that ensuring the right consensus is committed to ETH can be figured out with mutual staking orchestrated via smart contracts on both ends.

With zkrollups, there's a cryptographic guarantee that the off-chain computation is valid.

With optimistic rollups, there's a withdrawal delay so people have an opportunity to submit proof of fraud.

Bitcoin was always supposed to be a store of value, 21M and all that. The problem is that people think that store of value can't be a currency.

Nope.

I'm old enough to remember it was always called "currency".

A currency is certainly a store of value, how could you argue it’s not?