I wasn't sure I was going to chime in here but what the heck.
Versatile as in one "gun does it all", not switching uppers, not adding stocks or accurizing or free floating the barrel.
NO gun is that versatile.
My AK goes bang everytime but I don't expect it to be a sniper rifle and since its a .223 its not much of a deer gun either.
My 30-06 bolt gun is a hell of a elk rifle but I wouldn't dream of 'clearing a house' with it. In fact there is a far better chance at me shooting a 1000 yard match than clearing a house.
A milspec m-14 is NOT a mil spec M21 sniper rifle.
A mil spec M-4 doesn't have the velocity of a full sized rifle.
You have different tools in your tool box for a reason. Use the appropriate tool for the job.
Now lets face it when your government hands you a rifle you don't really have much choice and you 'make do' with what they give you. In WW2 that could be everything from a bolt action Springfield to a BAR. Sure a tommy gun was a great cqb weapon but at 200 yards it wasn't hitting anything. A carbine was never intended to be a front line weapon but it went everywhere in 3 wars. The M1 wasn't "the best" CQB rifle but it sure did a lot of CQB.
Military weapons development has been moving in this direction for years. You don't see regular troops with the array of weapons that were used in ww2. You see a single weapon and a machine gun. In that case the AR, AK, Galil, FAL, AUG, FN-MAS are all moving the same direction. They are all "sort of" filling these roles at once and in doing so aren't the "best" overall in any one field. The modern assault rifle is the weapon of a 'generalist' not a 'specialist'. Any modern assault rifle should fit the bill.
As far as the criteria you selected started with the exception of the 1000 meter match a mini 14 can fit the bill, but again your average out of the box rifle isn't a 1000 meter rifle either.
So is the ar the best? No but its a good compromise, the same as a lot of other modern paramilitary rifles.
on to part 2.. Yes there are a lot of parts available. There are far more parts available for amreican made weapons than imported ones. There are far more add ons available. Doesn't make them better or worse makes them available. Do you carry a spare parts roll? You know firing pin, a spring here and there?
Most of us aren't in law enforcement and we aren't in the military. If our rifles break we can go back to the truck and get another.
If you are in a shooting situation (ie people are shooting at me), I'd want a rifle that won't break and something to back it up with.
Whatever that rifle is to you, there are a lot of choices some more expensive than others. Pick one you trust and it can fill the role if it "has to".