When your balloon needs to lift 15 kg to 5 km, it suddenly becomes significantly expensive. According to the internet™, you need 14 cubic meters of helium, which alone would be like $700. You will also expect to lose half of them to random winds, so you need to launch at least 20.
Also, 15 kg drone is not that much destructive, it only carries 5 kg of payload, while Shahed typically carries 50 kg.