Somehow that's how it works with bitcoin but not cash, interesting. Despite the fact that cash is not actually that fungible (serial numbers, marked bills, digital bank ledgers) people use it fungibly quite often yet, given the same parameters bitcoin seems to fail your fungibility test. Why is that? It can't be tracking, ATMs read every serial number and which account withdrew them. The only difference I can see is that the ledger is public versus a private bank's SQL database. I agree that Monero has great fungibility, I dispute its network effect capabilities but that's an entirely different point.