how would a watchtower know if I'm losing all my cold storage coins?

Just like you have a watchtower for LN, you can have a watch-only wallet setup for your cold funds. Maybe you just have that in some app. Personally I have it in Core and it notifies me of all txs.

But what I do depends on the scenario of why I am about to lose them?

If your cold storage funds are p2tr and there's a weakness in it, consider it as if it's someone being able to spend every p2tr utxo. Similar to the bug in the TheDAO contract on Ethereum that infamously drove Vitalik to call for a trading stop and a roll back.

This is also why I am asking: is code law? You can technically amend your validation rules to reject every block containing a utxo spend of your wallet past block n (and thus be chaintip forking until the attacks stop) and if everyone would do that (non-centrally) then there could be organic consensus at some point.

I honestly expect that most people would just follow a central leader nowadays - i even fear that'd be Saylor if it'd happen tomorrow - and be exactly as centralized as Ethereum was and that everyone on bitcoin twitter (obv the same people that would follow Saylor) right now points to as why Eth is a centralized wefboi scam, but the optimist in me hopes to be wrong about that.