But I'm not sure this is really addressing the problems with risk management in large firms. My impression is that we don't really know what to simulate or what the tail risks are. Adding computers doesn't solve that problem.

Being that this has been my job for the last year, you're in my experience mostly right. The biggest problem up to now is in risk modeling, not in the application of the models, both in evaluating single risks and (especially) in doing risks aggregation. Many of the existing models for some common risks are also analytically solvable, and don't even need simulations to be performed.

For how much I like the OP's idea, I don't think that we are realistically at the level where such a proposal would give the best improvement to the current situation.