In a natural commons, the risk is that nature cannot keep up with reproduction to replace what is used.  However, in the steem commons, it could be completely depleted and the algorithms will continue to replenish the steem.  So it won't ever go away.  

The arguments being made about the rape of the reward pool is geared towards a single individual taking too much and the risk is that they grow so big that they can then end up taking the whole reward pool.  However, this argument falls apart when you end up with millions of people on the platform.  So while a few bad actors can profit heavily, the commons cannot be ruined unless the platform itself is abandoned by the users.  

So the economics are somewhat different than what is found in nature.