Existence of one stupidity does not allow for all stupidities. A perhaps unreasonably pessimistic assumption is that everything LV has done so far has been the correct choice, for reasons perhaps not well understood by Harry and thus the readers.

Regardless, the issue with the challenge is that if we can think of a solution, Voldemort is allowed t think of it unless it uses knowledge we know he doesn't have. The only other viable solutions are ones with no counter. This does have a counter (a very niche one, although I quite like the idea of an invisible death eater army and the visible one all being dummies; there are almost certainly other counters) and thus is not the optimal solution. I don't have a better one though as this is a horrendously high wall.