can you elaborate on this?

A 51% attack would cause enough pandemonium. A state actor could do that using technology the public already knows about.

The Bombe was used to break the encryption and read German army and navy cypher traffic. It was kept secret at all costs, up to and including sometimes _not using its results_ for fear of tipping off the Germans, even if this resulted in loss of life and shipping in the Atlantic.

Anyone far enough ahead to have a functional quantum computer powerful enough to mine all the Bitcoin, would instead keep it secret and use it to read foreign government encryption etc...

Emptying the blockchain into your bank account doesn't move the needle for anyone at that level.