It seems it depends on the concepts of the game being run by the DM.
If the conception is something like a Conan novel then your constrained to keep everyone weak enough for Conan to beat the crap out of them.
I for one like to go for realism in this regard. For campaigns it just improves my players ability to suspend their disbelief. It is much easier to think in terms of the real world. Yes, there are a vast majority out there that are simple farmers, tech guys, mechanics, etc... However there are also a lot of people out there gaining a lot of experience. These are what I refer to as the "movers and shakers". PC's fall into this group by default.
Greyhawk, Forgotten Realms, and most published settings all have numerous "movers and shakers". Some are even written up as NPC's in the setting books. I do not assume these NPC's are "frozen" until, or unless, they encounter the PC's. They are doing something, big or small, they are active much like the PC's.
So what is the players motivation in such a world? The same as it is in the real world. Wealth, power, morality, etc...
So just like people aspire to be the next Bill Gates, Eli Manning, Einstein, etc... the PC's aspire to be the next Elminster, Merchant King, Thieves Guild Master, Ruler of the World, etc...
How the PC relates to these others is measured by their level versus their level. Their resources versus the resources of their opponent.
There is no need to detail everyone, just having the assumption there are many adventurer types gaining XP's all the time established for your campaign is enough to make it work for when you do need to write up an NPC.
A really cool thing to do is a weekly newspaper. Go on the internet and copy and paste about a dozen news articles and then rewrite them to fit your campaign world. Insert names you create (Gygax's Extrordinary Book of Names, or any name generating website is great for this) and make sure to save these papers. Hand copies over to your players and recommend they read them. Then have PC's meet the town guard who was involved in the fight in last months bank robbery attempt, or meet someone who survived that big fire caused by the dragon attack, etc... Story hooks will explode from doing this newspaper, and having the PC's meet some of these NPC's will help with world immersion in a big way. Being able to "copy and paste" most of the story helps speed up the process. By your 3rd issue of your paper you should be zooming through it.
Adding all of these extra little touches adds depth to your campaign world but also helps reinforce that your world is dynamic and that lots of other people are doing things too, and gaining levels.
plus players get a big kick out of it when things their PC's do make the newspaper. Especially if they are head line news. Except for the Rogue, they usually are upset that their identity is so public.