There are financial holdings worth 100+ times that bitcoin wallet. Likely protected by a lot less than 100 times the security of a 256 bit key. Apple's hedge fund, Braeburn Capital, likely qualifies. It's arguably the largest hedge fund in the world. Gain access to their network and you could wreak serious financial havoc.
It is probably the case that there are multiple cryptographically-secured security layers standing between an anonymous internet user and the innermost layer of such a system.
Rather, stuxnet comes to mind. We're talking about $150 billion after all.
Of course the other issue is, I'm betting they're holding a lot of short term government securities. It simply isn't plausible to steal $70 billion worth of short term US paper and do anything with it.