It’s *way* more than 5%! A CRQC operated by a private entity will almost certainly not be interested in stealing 5% of the supply and sitting on it, they’ll likely want to sell a decent chunk of their stolen coins to pay back investors for the immense R&D cost they spent. The total quantity of coins available on markets is not anywhere close to 20M, it’s a tiny fraction. Having something even like 1-2% of total Bitcoin supply flood the market at once is going to have a very large impact on price.

As for your claim that this is somehow changing a fundamental property of Bitcoin, i think you’re losing the Bitcoin philosophy for the way it happened to be written down. Yes, it’s critical for Bitcoin to have a hard line in the sand against coin theft. But you don’t get to pick here - the coins are going to be stolen or frozen no matter what you do. Getting myopic about *who* is doing it isn’t a part of Bitcoin’s value proposition, you’re just reading too much into the way the rules happened to be written down, not the reason for them.