Lightly modified FPVs with grenades are a major concern for soldiers, but so are mortars and artillery shells etc. There’s a lot of low cost long range weapons vs infantry, but a drone with a grenade isn’t really effective vs tanks etc. A fleet of cheap drones just don’t do anything if bomber aircraft can simply fly higher than they can reach.
Leave the realm of mass production and you can build drones that would be, but they quickly start looking like existing systems because militaries have been working with drones since WWII.