Good call, here cannabis damaging effects are usually minimised and hidden under the rug.
And you will always, always find someone telling you “what about alcohol”
Eh, "I don't believe your claimed reasons because your actions don't seem consistent with them" can be a valid things to bring up. Of course it becomes less so if you then refuse to hear any explanations that aren't the one you came up with.
But it's not an argument to prove that weed is harmless. Its definitely not.