The government doesn't need to go beyond finding the tumbler. Here's how a sample investigation would go:
Honeypot drug purchase, paid via bitcoin to DumbSeller. DumbSeller routes bitcoins to tumbler. Using the NSA backdoor in TOR, they trace DumbSeller to VPN service, who they then subpoena. VPN service either complies or gets charged as accomplice to money laundering and other crimes and gives up customer as part of plea deal. They then trace the payment to VPN until they reach a warm body at the other end.
Let me give you some more parsimonious paranoia: the tumbler operator is likely already working for the government, or accepting money from governments to forward them DumbSeller's transaction history. Why would they not?
Running a tumbler seems like a great racket if you can cope with the amorality (and particularly if you can rationalize the fact that you're sending money to ISIS with probability approaching 1). People pay you fees in exchange for "privacy", and have no way to tell if they got it.
If there exist tumblers that don't sell their customers' transaction history, in the ruthless capitalist world of Bitcoin, they are easily outcompeted by the ones that do. And a tumbler "customer" has no way to shop around for privacy.
Honeypot drug purchase, paid via bitcoin to DumbSeller. DumbSeller routes bitcoins to tumbler. Using the NSA backdoor in TOR, they trace DumbSeller to VPN service, who they then subpoena. VPN service either complies or gets charged as accomplice to money laundering and other crimes and gives up customer as part of plea deal. They then trace the payment to VPN until they reach a warm body at the other end.