[ConspiracyFact]

"Who will watch the watchers?"

There is no good answer--I agree with you about the infinite regress--but there is a counter: the first term of the regress often offers a huge improvement over zero terms, even if perfection isn't achieved with any finite number of terms.

Who will stop the government from oppressing the people? There's no good answer to this either, but some rudimentary form of government--a single term in the regress--is much better than pure anarchy. (Of course, anarchists will disagree, but that's beside the point.)

Who's to say that my C compiler isn't designed to inject malware into every program I write, in a non-detectable way ("trusting trust")? No one, but doing a code review is far better than doing nothing.

What if the md5sum value itself is corrupted during data transfer? Possible, but we'll still catch 99.9999% of cases of data corruption using checksums.

Etc., etc.