You could strengthen this argument by saying counterfeit monero would be like pyrite or fool's gold, the difference is we don't know if counterfeit monero exists. You can get gold for free without working for it (owning a mine) but someone still has to do the work (mining). Pyrite is a way to bypass the gold mining entirely - false proof of work. It's contingent on the average trader not knowing how to distinguish it from the real deal. It doesn't stop gold from working as money because the more you use it, the closer you get to being found out, which is always inevitable after enough time.

People using gold before they knew about pyrite didn't stop using it when they found out about pyrite, so Monero doesn't have to go to zero if counterfeit Monero is discovered. Someday it would be like pyrite or fool's gold, where it's been around for a long time and it's common knowledge and you can't get away with trying to use it in place of the real deal at all.