"Guys, we are going to start quantum exploiting your blockchain for profit in a couple of months, please fork before that XOXO" - No criminal ever More likely:

"Guys, we are going to start quantum exploiting your blockchain for profit in a couple of months, please fork before that XOXO" - No criminal ever 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".