So, the practice of over-sampling (if you will) has no downside risk. None. The bootstrapping issue you raise doesn't exist for the investigators. They have to have suspicion before they can access your NSA file, and armed with suspicion, they can easily get a warrant.
“They have to have suspicion before they can access your NSA file”
Who is “they”? Snowden had access to all the data. Many others probably have access and are willing to sell it to the highest bidder. Then you also have hackers and corrupt politicians.
Never assume that a government program will work as it’s supposed to (ie. in compliance with the law). Because it never does.