Please define your new take in the interpretation of the word "sole".
The actual sole purpose of what most people refer to as an "assault rifle" is just to be a modern, reliable, modular platform that can be customized to fit the needs and use cases of the owner. It's good at that, and so it's good at being customized for a lot of different uses.
The hunting argument you make is dumb. You would need to turn around and argue that any advancement of any produce anywhere that allows it to perform even marginally better than absolutely necessary needs to be undone. The fastest posted speed limit in the united states is 85mph, and yet every modern vehicle can exceed that by a lot...some of them by double. It doesn't mean the sole purpose of the car is to break speed limits.
If you break it down by time used for any one specific purpose, then the primary use case of an assault weapon is to be stored in a box or a case, unused (that is what the vast majority are doing the vast majority of time). I would argue the primary purpose is synonymous to the use case of an insurance policy (something you have in case you need it but don't actually ever use it). The next most common use (by time spent performing in the role) is to exist solely as a show-of-force without even being fired -and that seems to work pretty well because just imagining the appearance of one tends to get people upset and agitated. For the rifles that actually get used regularly, practice is another common use (using it to maintain proficiency with marksmanship skills) and also shooting for fun (which isn't always/necessarily practice) is a common use case. In the past, I have used mine for both hunting and for protection against potentially dangerous wile animals while hiking through the vast wilderness of the pacific northwest - I personally don't like the idea of having to mess around with a clumsy bolt action in the event I might need to fire multiple shots.
From the gun manufacturer's perspective, the 'sole purpose' of "assault rifles" isn't to "kill people as fast as possible", it's to: sell weapons and make profit. The "sole purpose" of a thing is defined by the user...and at least in the united states that means a lot of things other than killing people.
Draft dodging piece of shit tries telling me, a left leaning veteran of the US Marine Corps, that I am vermin...on veteran's day. Yeah, get fucked, asshole.