Years later, Colt made a sem-auto version for the civilian market and, since they owned the rights to the name "AR-15", sold it as the AR-15 to differentiate it from the full auto M-16.
People buy the AR15 BECAUSE it is the closest thing they can buy to an M16.
If people could buy the M16, the AR wouldn't be nearly as popular.
What is the demographic of an AR15 buyer?
I think you are wrong about that. Full auto has little practicality in anything other than full combat. In civilian use, the novelty gets old and expensive really fast.
Anyone who wants to get the thrill of full auto can do so for much less money with a bumpstock, hellfire trigger or even with the rubber band trick.