I'm not sure how this is a counter to the human analogy? I'm assuming you're assuming probes reproduce and propagate perfectly, but if we assume a 'human reproductive unit' (be it a pregnant individual, a couple, a minimum viable population, whatever) has perfect genetics which can be perfectly replicated, then 'it just takes one' as well. While some sort of b-vn scenario seems plausible, at this point in technological development the practical contingencies of such a scheme are pretty up in the air. I mean, look at it from the other end. If we assume arbitrary technological progress, it's possible probes are everywhere, but we wouldn't even recognize them as technological artifacts if we were starring directly at them.