Thank you for the time and effort, and I will be looking to your explanation, but I have an immediate problem.
I have never read Moorcock or Conan either. So, you are comparing a thing I don't know except through references to a thing I don't know except through references.
Which might explain the problem, to a degree (I have also never read Song of Fire and Ice, or watched Game of Thrones, so going to that explanation would also not help make it any more clear.)
And this gets me to the second problem I seem to have. Much like
@Helldritch talking about making the setting gritty and other such things, what you have described is a style of play.
FR is a setting in decline too. The greatest ages of magic are behind them. Gritty anti-heroes adventuring for profit and not for the good of the world? In all the FR games that I have been in (because I have a DM or two who use the realms since the APs are set there and the maps are easy to find) that's exactly what people did (I am too much of a goody-twoshoes, but I tried)
You could trivially set an adventure in (pulls up map, picks random point) The Border Kingdoms near the Shaar Desolation, and have a gritty survival game where you are the hard, uncompromising hero who is a better alternative than the evils he fights.
So... what gives? What makes Greyhawk as a setting something I'd want to run six different styles of campaign in, if the only hooks are "well you can run this one style of campaign really good, because it matches the history of the world"?
And to state again, I find FR boring as crap. It has a few interesting ideas, but nothing in FR really strikes me as highly compelling either. Though I think that is in part because it is so frickin big, and every game I've ever been in focused on one of two cities (Neverwinter or Waterdeep) and we never went more than a week's travel into generic dungeon beyond that. But, all people seem to be able to say to advocate Greyhawk above FR is that you can run a gritty game where people aren't heroes.... and I can do that in FR, or Eberron, or Wildemount, or Ravinca. So the hook really reads to me as "You can run the game you can run in any setting, but this one is Greyhawk"
And, I suspect that people are going to say you can't really run a gritty game in a setting like Eberron or Ravinca, because there is too much magic. But, in my experience, there are always plenty of sections on the edges of the maps that are harsher than the centers. Sure, House Jorasco has Healing Houses that can cast cure wounds.... but like all hospitals, they exist where the customers are, and maybe out in the border between Breland and Droaam, there is no profit and Jorasco healing is only available if you make a week long trip to a major city.
And, the advantage of a setting like that, is that you can run both games in the same setting. Shifting from a border town in the middle of nowhere to a major metropolis with vast resources lets you play both styles of game, without having to switch settings. Something you can't do if the resources simply never existed in the world. And sure, we could talk about how "X high level spell means you can trivially travel to take advantage of those resources" but you can't really have a properly gritty game when people can just teleport wherever they want anyways, and a lack of magical resources means that your PCs are suddenly scarier than anything else in the world, because nothing else has access to the capabilities the party does.