I completely agree, which comes back to why I disagree ;-). Trying to do a whole world justice is all but impossible, trying to make a whole planet one thing is preposterous. For a campaign setting (as opposed to a campaign world) I would prefer to simply be focused. you need outside influences for that focused setting to work, you can't pretend those things don't exist, but you can sideline them. 7th Sea is an excellent example of a setting that frustrated me, they wanted age of sail, so they had it. They didn't ask what was necessary for it to make sense (no Cape of Good Hope + Indian Spice trade = no 17th century sailing vessels), they just hand-waved it without a single explanation for why these people would sail at all.
You sound like a man after my own heart. So, briefly, yes, trying to give treatment to everything is too ambitious. I'm not suggesting that. I am suggesting that thought has to be given to the world in its broad outline before you can even reasonably establish a single cultural entity. As you say, very distant parts of the world can impact the culture and economy of bits and peices of it. But I'd never suggest you try to detail all of it, just that your broad conceptions made some sort of passable sense.
I never found FR particularly useful. It had lots of detail, but none of the detail I wanted. I consider FR the archetypal bad campaign setting.
That said, let me fork completely off topic. In my own homebrew, my characters are currently in Talernga on the eastern coast of Sartha. Talergna is the major intellectual and financial hub of the region, historically as the gateway to Sartha and also lynchpin of the entire Storm Coast trade (the major north/south trading route for goods in that part of the world). Way on the other side of the equator to the southwest, there is a nation I haven't done much with ruled by an immortal god king, and loosely based on a romanticized sub-saharan African kingdom - a medieval Benin on a grand scale. Now, this conception dates back to middle school, and lately I've been thinking, "What sort of commercialism does this ostencibly wealthy kingdom run on? Does it have any reason to trade with the northern regions of Sartha?" What prompted this thought was I was going to have an ivory dealer from the far south in Talernga, but then I realized - Talernga doesn't need imported ivory. I've already established the existence of domesticated mastadons along the Sword Coast, and there are wild herds of both mastadon and mammoth elsewhere in the region. So, what would drive long distance trade, and even more critically, what would you exchange? Much more emphasis is put on what was coming back from the far east than what was being used to purchase it. Was it just European silver flowing east in return for all those 'exotic' commodity items? If you were a European trader on the outbound voyage where you moving goods in short hops eastward between the intervening cultures, or did you have some commodity in addition to coin that you were taking the distance? Conversely, what might a eastern entrepreneur headed to Europe be hoping to buy? I can't think of anything really other than glassware, but that opens up another can of worms - if we accept a glass mania in our 'spice' region that is trading for glass, how does that impact the arts and architecture of our 'spice' region? I'd rather have more going than glass, and more balanced intercontinental trade.
Feel free to fork if you want to essay.