I think you're at least somewhat correct in your assessment if we are only talking rifles. I think a significant portion of handguns are purchased with SD at least somewhat in mind. (And by definition if intended for carry)Most guns purchases are not for self-defensive. I think more guns are bought by hobbyists or hunters.
In fact, I would argue that a big dog is a better defense mechanism than a gun. Barking has been proven to scare away most attacks/burglars and prevent encounters in the first place. You are also a lot less likely to face any penalties if a dog bites/harms an attacker versus if you directly shoot them.
Dogs are great but off the table entirely for a lot of people for any number reasons. Even a little dog that would be almost useless as a "guard" dog can be useful as an "alarm" dog.