The case against koalas

There was a reason 18th-century rulers were eager for their subjects to grow and eat potatoes: the miraculous tuber offered an alternative source of nutrition to grain, hence reducing bread prices. In the event of a catastrophic harvest, people could survive. To the rulers themselves, however, the biggest benefit was probably what happened when the grain harvest was merely disappointing. With grain no longer critical to survival, the price of bread would be far less volatile. And high bread prices might be more likely to lead to civil unrest than no bread at all.

Humans evolved to be foraging omnivores, but agriculture made us over-reliant on whatever crop could best be grown nearby. The potato rebalanced that. Being a monovore is never a good plan in the long term. Notice that koalas, despite having opposable thumbs, have never successfully colonised distant continents or developed extensive worldwide trade networks. Had they done so, it is likely that eucalyptus leaf prices would have been very volatile indeed, quite possibly manipulated by some shady koala cartel.