When you have a *bitcoin* TXn buried under 100 blocks you can be damn
sure that money is yours - but only because the rules for interpreting
data in the blockchain are publicly documented and (hopefully)
immutable. If they're mutable then the PoW alone gives me no confidence
that the money is really mine, and we're left with a much less useful
system. This should be more sacred than the 21m limit.

Well, I think we should avoid the term "sacred" - nothing is sacred because we're not building a religion here, we're engineering a tool.

Consider a world in which 1 satoshi is too valuable to represent some kinds of transactions, so those transactions stop happening even though we all agree they're useful. The obvious solution is to change the rules so there can be 210 million coins and 10x everyones UTXOs at some pre-agreed flag day. We probably wouldn't phrase it like that, it's easier for people to imagine what's happening if it's phrased as "adding more places after the decimal point" or something, but at the protocol level coins are represented using integers, so it'd have to be implemented as a multiply.

Would this be a violation of the social contract? A violation of all that is sacred? I don't think so, it'd just be sensible engineering and there'd be strong consensus for that exactly because 21 million is so arbitrary. If all balances and prices multiply 100-fold overnight, no wealth is reallocated which would be the actual violation of the social contract: we just get more resolution for setting prices.

So. The thing that protects your money from confiscation is not proof of work. PoW is just a database synchronisation mechanism. The thing that protects your money from confiscation is a strong group consensus that theft is bad. But that's a social rule, not a mathematical rule.