If unsure about future prohibition worries, just buy a lower reciever for a 100$. If everyone gets the knock to turn them over one day in the future, its only a 100$ loss. If they get grandfathered to the owners in the future we still might be able to use them at the range and the lower just might turn itself into a gun for use.
In the meantime now is the time to buy them. The AR glut is slowly drying up and prices are only going up. The S&W Sporter will make an excellent entry AR15 for a what he said newbie who doesn't want to spend the big coin.