But the wider point is generally that just because something is less effective doesn't make it useless, and just because something is effective doesn't make it dominating.
If an enemy has an artillery advantage, then shelling obvious decoys is still taking decoys off the field, which you now need to replace. But worse, their existence is giving away the fact you're active in the area, and their placement is giving away your operational range - i.e. how far can a person move on foot over rough terrain? How far in a vehicle? etc. What's the effective range of their normal infantry weapons - if you know there's a decoy then the trap has a specific radius if it is a trap.
All on the bet that they will in fact run out of shells - or in the case of drones, they won't even run out since a drone can much more easily be re-targeted.