The feds were able to link them to the couple because bitcoins were moved from VCE 4 to other accounts that also had received bitcoins which were traceable to their alphabay accounts.
If they had funneled everything through monero I can't say that they wouldn't have got caught like this.
Nobody knows who Satoshi was, everybody just assumes he isn't the feds themselves. Such scenario would be so catastrophic to organizations that rely on its advertised anonymity and it would be the best investment ever since you could just keep denying you have this capability, always coming up with a cover story (keys in cloud, we traced their keys, they were dumb etc).
This would keep criminals guessing, the economic value of laundering with crypto is too great to give it up but at the end of the day, they can only operate under uncertainty.