But see, that's where I disagree right off the bat. A commoner isn't equally good/bad in every aspect of their life. Each person is a mix of stats, so to say a commoner is centered around an average of 10 is reasonably, but not all 10s. Certainly a blacksmith is stronger than most clergy, for example.
I also don't see any reason why a system can't be designed that acknowledges the world and not just the PCs. If you want the PCs to be above average, or different than the rest of the population, then you can design around that.
But a soldier might not. Perhaps the training is sufficient to warrant a higher Strength than the rest of the population. But I'm not so sure that military training as a whole reflects this. English Longbowman, sure, that was a specific type of long-term training. But your average soldier conscripted to a pike and shield wall? Probably not.
Agreed on what the DM can do when creating an NPC. My goal isn't necessarily to roll for every NPC, etc. It's more that I prefer a system that can be used for that if desired. Really, it's a matter of defining what result you want in terms of stats, and the system is designed to support that. If I know that the system is designed to average 72 points, then I can use that as a guideline for creating an NPC, and make a more average individual closer to 63, and an exceptional person 75 to 80.
A commoner is an NPC the PC dont care about. Do we have to know that the Blacksmith have 13.564 strength!
DnD is not a sim game. Population, economics, politic need only sketchy rules.
But if you choose to make a particular commoner have three 18 stat, he is no longer a "commoner".
He is rather a future hero, and the PC should sign him right away!