The better retort would be to argue that the as the adoption increases, the price will rise so the fixed size (in coins) tail reward has an adaptive valuation.
But I will retort that the value of shorting also scales up accordingly.
Shorting can't erase the cost due to PoW (burning energy). It can only erase a cost from loss of value of a holdings (PoS and other methods that claim to turn holding coins into "virtual miners").
If attacking a coin causes its price to decline, shorting can return a profit. If that profit exceeds the cost due to PoW, then that cost was erased. Cover the short, stop the attack. Repeat if the price rises again.
BTW, I would suggest that Tragedy of the Commons is an ineffective analogy for explaining whatever it is you are trying to explain because obviously-intelligent people such as ArticMine don't understand it. It may be that you are entirely correct, but if you want to communicate effectively you need a differently-worded explanation.
Agreed at the appropriate time. I deem it necessary to be vague since I am months (or moar!) away from implementing my design.