More likely: "We'll skim from dormant wallets that won't be immediately noticed, and when it becomes too obvious, we move all of Satoshi's bitcoins to force an emergency fork that launders our profits".

I could exploit the old blockchain, then migrate my stolen keys to the quantum blockchain.

Forking doesn't work here. Traditional computing and Quantum computing are different enough that "porting" isn't just something you can do.

It's a question of how much time a new algorithm buys.