These are not existential risks, they are real security issues. If there is not even a meaningful definition of Bitcoin's security -- a definition that can be used in a mathematical proof, without circular logic -- then how can Coinbase make any kind of claim about the security of their own system? To put it another way, suppose Bitcoin is inherently insecure -- suppose that nothing you do can prevent a polynomial time attack (this is not unheard of in crypto; see Merkle Puzzles) -- can you even claim that Coinbase is secure?