You're right.
But...
Where are you gonna get more 308 (presuming...) in that event? You won'd be getting it at Walmart or local gun shops. They will be closed, long looted clean of anything remotely useful, or possibly heavily fortified in the time it would take to go through 1000 rounds.
I don't know where you were picking up those AK's (Vietnam, Somalia, Iraq, Afghanistan, et al) but I doubt their mags and/or ammo fit in whatever rifle you were carrying as a US soldier, sailor, airman or Marine. In that case, you didn't get a resupply, you got a replacement firearm. That alone makes your entire argument irrelevant.
In the event of "SHTF" and you are shooting so many zombies or commies or whatever that you run out of ammo and need to pick up ammo from [insert location or dead gun wielder here] you can also pick up a weapon and be back in business.
So after the zombies eat their way through the National Guard, I'll be picking up an M249 and as much ammo as I can squish into my zombie bug out bag after dumping out all my toilet paper, food and porno mags. I'll keep my water though. 50,000 rounds of belted 5.56 is heavy and I would like to stay hydrated. I see no reason to carry any less, since I have already gone through the 1000 I had at the beginning...
You will NEVER have a need to forage for ammo. If you're out and you can't buy more, you're out.
Moving on...