Then, when your laptop gets nicked, you use the key in the safe to move the coins. And vice-versa if the safe gets broken into. Of course there are failure-modes:
1) If BOTH your laptop and your safe get nicked you're effed.
2) If the thief spends the coins before you realise they're gone, you're effed.
3) You can't do all this retroactively. If the thief's already stolen your coins protected with a single private key, there's nothing you can do about that NOW, just do the multisig for your future coins.
I'm just trying to wrap my head around this, but wouldn't it be the same if you just printed your only private key on paper and kept it in a safe?
That way, when a thief nicks your laptop, you open your safe, scan your paper, import your key and move your
BTC to a new wallet before the thief figures out what that "bitcoin core"-program on your laptop is doing?