Remember the key here is that in the swarm the drones coordinate their efforts and movements with each other. It would be too easy for an “enemy drone” to be taken out before the entire swarm of 100s are all taken out. Not only that it is very easy to keep adding 100 more to the swarm to join the “fight”.
Coordinating movements and responding quickly in an intelligent manner are not trivial tasks. All manner of countermeasure could be developed. Some of them would be really cheap.
It is one thing to win a war with drone swarm, but may be another to effectively enforce occupation.