Zombies can and do drag themselves, or even worm their way across fields. Until the brain is destroyed, they’re a threat.
Head shots are much easier when zombies are moving slower. I suspect a platoon with 8 guys shooting an M2 and the rest with scoped rifles could easily protect an encampment from thousands of zombies.
The study found a finite number of monkeys in a finite amount of time would not write all the works of Shakespeare.