FWIW, I work on Google Search, possibly the largest software system on the planet, and Google is famous for being tester-light and automation-heavy.  We rely much more on code reviews and on having good engineers who aren't rushed than on testers.<p>The reason is just as you state: beyond a certain point, the number of states in the system becomes so great that you simply <i>can't</i> test them all.  Replacing developers with testers doesn't change that.  So you're better off understanding what you're doing than trying to avoid all the possible ways that you could go wrong.