The same adversarial system that would be used for Snowden, and was used for Swartz, Ellsberg, Drake, and even DJB?

Admittedly that's kind of a strawman as you're not claiming here anything to do with those one way or another. But many who don't trust the government to put people on trial do point to cases exactly like those.

On the other hand if you're simply saying that we shouldn't trust government to prosecute cases, operate surveillance without oversight, etc. just because they're the government I would agree with you 100%.

But on the other hand by the definition I wouldn't even trust the government to deliver clean water or build roads without oversight. As an example of where even the latter could go wrong I'll point to a book that HN pointed out to me, "Fatal Purity" about the French Revolution... one of the many examples of then-current corruption by the nobles was of road-building. The specific example was of a new provincial road that was argued necessary for the people to build despite there being an existing road... it was built right up to a nobleman's estate and then the rest was canceled, leaving the lord with a gratis road built straight to his property.

Or for a modern example, the famed "Bridge to Nowhere".

So there is no part of government which is inherently trustworthy and worthy of operating without oversight. But once you've established good oversight (such as you'd definitely want over criminal prosecutors!), that leaves the possibility of other government-provided functions open again.

There may certainly still be reasons that government should not perform those functions, but it wouldn't be about the impossibility of providing oversight.