>Nobody knows exactly how often Bitcoin is used for payments or what it's used to buy.

I wouldn't go that far. This is much more related to Bitcoin's relative "newness" and lack of mature AML tools than anything inherent in the protocol. There are a number of startups that offer blockchain tracking and forensic analysis capable of tracking the kind of money flows you describe. Your statement seems much more applicable to something like Monero or other obfuscated ledger intentionally designed for privacy.

It is an open ledger, after all.