It’s not original though. Classic, trite precoiner FUD that’s exploited by shitcoin scammers to justify multicoinism. Alex learning in public will hopefully be a net positive for newcomers.
Discussion
Having more than one currency doesn’t need to be justified. Having only one does.
It’s the exact opposite. More than one is inefficient and regressive towards barter, but the century of authoritarian propaganda and recency bias convince people that multiple is the norm. It’s not.
Definitionally, money is the most salable good, so there can only be one. Reality might approach that imperfectly and have a few competing options for some reason.
I think it’s less how many currencies there are that needs justification, and more the reason there is either one or more than one. If it’s something like regional or cultural norms, okay. But if it’s because some top-down entity said so (e.g., fiat legal tender (one) or bimetallism (plural)) — that’s what I think needs to be (and can’t be) justified.
With physical currency, gold is the leader, but there is also silver, important because it has a high commodity value from silverware to batteries (to film back in the day), and you need both. And then there are less used metals like platinum (hard to work) or copper. Notice that original US constitutional currency had gold, silver, and copper. This has been the case for thousands of years.
If you want an arbitrary analogy:
Bitcoin = gold
Namecoin = silver (provides globally consistent namespace)
Pktcoin = copper (provides metered income from amateur internet links)