Such contracts would not really add any net costs to the system, since they cost virtually no resources. Whereas if I send $100 to the winner of a PoW contest,

Such contracts would not really add any net costs to the system, since they cost virtually no resources. Whereas if I send $100 to the winner of a PoW contest, then $99 worth of electricity will be expended. Even if the contracts did add some costs to the system, PoW systems can have similar contracts. I could join a PoW pool and agree to share any $100 rewards I receive with the pool. Alternatively, we could just avoid the matter of contracts by replacing $100 with $1. $1 is small enough that nobody would need to think about diminishing returns, so there would be little point in spreading out the risk.