I think your argument is an interesting one against tokens. But not the authenticator bag slots. The authenticator is free. You cannot purchase it, it is free. There may be a limited number of people without a smart phone who also have access to a computer that can play WoW. I don’t know what to say to that except that whatever issues you have going on that resulted in this particular combination are probably more important for you to focus on than whether or not you have 4 extra slots in your backpack.
The bag slots are in your backpack. Securing your account benefits you and the whole classic community. Since the authenticator is free, then the value of the bag slots is 0g. Also you cannot sell your bagslots to someone else – value 0g.