They were designed to be a general issue infantry rifle that has been pressed into many different uses and configurations over the last 60 years.
You'd have to go look through Eugene Stoner's papers to see if he left any record of what he designed the AR-15 to be. Or consult a medium.
What the Govt first bought them for was not as a general issue infantry rifle. Gen LeMay bought some for his Air Force SAC guards, to replace the M1 carbines they had. The Army was dropping the carbine, and the Air Force got its small arms and support parts from the Army, so LeMay was going to be "out in the cold" holding on to the carbine and found a superior replacement.
It was the "whiz kids" at the MacNamara defense dept that decided it would be a general issue infantry rifle, whether the Army wanted it, or not. There was a lot of pushback for some time, but they made it stick and here we are today with multiple variants to better fill various roles.
I wonder how many of the ones standing in line to get a firearm were anti-gunners just a week or so ago??
Probably not too many, yet. Spoke to a friend today who works at a shop and yes LOTS of people who never before were interested are buying guns.
I think the "Converted" anti gunners will show up in droves IF civil unrest reaches violent levels. The irony is, IF it gets to that point, they probably won't be able to get at gun, legally. Even worse, if they can, they won't know much, if anything about how (and WHEN, and when NOT TO) to use it.