This breaks down when multi-term policy decisions need to be made. This would only push them to kick the can down the road
Also doesn't capture in the the unequalness of gains. Which is better - the fed doing something that has 90% chance of huge gains or a 10% chance of catastrophic economy shattering loss? Or an 80% chance of huge gains and a 20% chance of gains netting 0? The first is much more desirable if maximizing a bonus and could be tempting - but I dont think we want that risk.