Argue that TH7s should be able to level another TH7 with mass dragons all you'd like, but don't try and make the case that there's ever a situation in which mass dragons won't 3-star another TH7. If you use 3 lightnings to destroy the more difficult to reach AD, you can level the entire base with 5 dragons. When I was a TH7, I 3-starred TH7s with 10 dragons without using any spells. Heck, on a couple occasions I closed my eyes and just rolled my face on the screen and still got the 100% win. If you are willing to spend the elixir, they are a guaranteed 3-star.
To the OP: what you're suggesting basically just ups the damage of ADs, given they are attacking dragons. Nobody would not set them to dragons as the favorite target, as they'd still hit other flying troops (if the raider even used any other flying troops in his/her army) once they take down whatever dragon(s) are in range.
What you need is some risk/reward with your idea. What if you could set an AD to target dragons at double the damage, but then they didn't attack any other flying unit? It would be hidden from the attackers, meaning they couldn't click on it and see what the setting was, unlike inferno towers/xbows, but during war they would learn fairly quickly which ones are "anti-dragon ADs" vs regular ADs. It would make those bases more vulnerable to balloon raids, giant-healers, etc (this is the risk in the situation), but maybe not until after a failed dragon raid or two were committed to that particular base.

