But in the context of bitcoin, it seems likely that if the bitcoin client originally had such a problem, someone would have already exploited it to steal funds.

But in the context of bitcoin, it seems likely that if the bitcoin client originally had such a problem, someone would have already exploited it to steal funds. All the old funds, mined with the original version of the bitcoin software, are still just waiting there at the original addresses. Especially in light of the creative crypto attacks people have done on other implementations (like the one where attackers stole from android wallets based on a flaw in the system-wide android PRNG). Money is a powerful motivator.