The short version is that the supreme god, Ao, elevates extremely flawed mortals into gods then demands that people worship these ascended mortals on pain of eternal torment. Note: as written this includes every child who dies before the age of reason....
You know, as much as I consider Faerun theology to be unholy bloated mess, and I would simplify shit out of it, just to make it somehow coherent (goodbye 2000 gods, welcome like one pantheon, with multiple racial interpretations and masks), generally I think it fits kitchenk-sinkey pagan-pantheistic vibe (although as I said too messy). You say its not Western. Well yes. Or rather. Notion of Good is kinda Western, but because it's D&D then Good and Evil are equals, and Evil is not mere corruption. It stands in its own right. That's literally Hella difference.
I'm quite sure some aspects I remembered quite differently - Wall of Faithless is not eternal torment, you dissolve into Wall just like you dissolve in your chosen divine sphere. And since Kelemvor took over, while he was forced to keep the Wall by rest of pantheon, he made it AFAIR painless. Though well... boring.
Another thing is - divinities are not generally ascended mortals. They are few ascendants who replaced dead or retired gods, but even they took their power from ancient primordial spirits in proper sense. Shar, Selune, Tempus, Jergal, Mystral, Helm, Talos - they are all well definitely not-mortals. Though of course they are limited and flawed. Like you know pagan gods in mythologies, what I can say.
And while sure each soul of Faithful shall land in domain of their divinity... I'd say even though it's just for 100 years, there is vast difference in quality between landing in Celestia or landing in Abyss. I doubt Talos is nice host even to own worshippers. (And I mean forces of Hell are all over punishing damned they corrupted in first place, so go figure).
And AFAIK it was the same in other D&D settings so I think we can argue that overall D&D nature of soul is such it can contain mortal indiviuality only in limited time and it will return to soulstuff it was taken from in Great Cosmic Cycle of Recycling. Alas you can decide which part of eternal and equal forces you gonna help with this pinch of soulstuff, before ultimate demise (and whether this ultimate demise will happen in nice retirment home, or in hellhole full of angry and hungry daemon-bugs. So overall I think that dunno escaping to Greyhawk or other sphere won't help. Soul gonna do soul-thing. Alas among major religions of world is still Buddhism who believes into generally self-anihilation of self and impermanence of soul. So it seems on practical level human beings can survive with theology without promise of proper even after. From three great setting of D&D: FR, Greyhawk, Eberron - neither consider human soul to be immortal monad. So I'm gonna assume it's cosmic universal truth among Spelljamer spheres, and all worlds Planescape is linked for. (Honestly that's at least one cosmic consistent thing.)
* Eberron’s while not piecemeal is basically the result of the same type of failures… specifically, in trying to devise an afterlife based pre-Christian understandings they didn’t look beyond the grey mists of Hades and so didn’t bother to include the Elysian Fields or Tartarus in the mix… everyone just ends up a disembodied spirit wandering without memories through an endless grey dimension… so again, zero moral structure to encourage goodness among mortals instead of indulging your every whim while you live (and every horror you commit to extend your life rewarding you with no metaphysical downside for your eternal soul) other than the generally proved false by human history belief that most people are intrinsically good (some surely are, but again… the logical end point of any moral system without just rewards the righteous and punishme for the wicked is going to end up with the majority following an ethos of “might makes right/whatever I can get away with”).
Well yes, but most people in history did not abandon wicked ways because of promise of divine reward. Most did it because society to function need to enforce some morale. Those that did not collapse into anarchy. Even warmongering violent societies had/have social rules to minimize infighting. That's matter of survival. And if we look at many non-Abrahamic religions in history - yeah they worshipped cool gods aside of asshole gods, and so on. Sure real human nature - and therefore most fictional ones - is Fallen, but is not total fall. "Might make right" can take you even on practical level only that far.
And even with greek Afterlife. Elysian Fields were like for 0,1% of famous heroes, and Tartarus was not simply for assholes (at least not eternal Tartarus) but for people who basically broke most holy taboos or insulted deities itself in grevious manner. Basically all examples of condemned to Tartarus are like WOAH. Common murder... come on, who cares. Definitely not Ares.
So yeah for most of faithful of pagan gods, be it Indoeuropean or Semitic religion, and many others, afterlife was slow dissipation in grey mist, unless you were great hero or supervillain.
Then of course my practical experience with Faerun - both by Baldur's Gate trilogy, and by practical roleplaying never gave me feel I play in anything resembling Western society in terms of faith and philosophy. There could be western technological trappings like knights in full armour - but that's technology. Economic necessity. Some sort of feudalism would probably arise in Germanic Europe even without baptism, as matter of necessity, and military evolution could very likely be simmilar. I mean what D&D did to paladin while grevious, it's still less they did with Celtic pagan - druid and bard
I mean where are my 400 sacred poems! But on ideological level? Nah. I mean all divine stuff, and religious cults were weird and more from Conan books than medieval Church, and that's probably proper.
So ultimately from human perspective (and bloat aside), I think most of mankind would generally accept reality, and live as they lived, because ultimately nature of most is centred on survival, and not on transcendence unfortunately. I understand your sentiment, I can even share it alas I don't think it would matter to actual human population if they were living in such world. Even most of theologically inclined - would probably accept this model as natural law. I mean many ancient philosophers did. So ultimately... for my Rasheman barbarian I don't think it matters much. What matter is glory to gain, as only glory is immortal.
But I look at Golarion the same way.
Well at least in Golarion I think there is stronger assumption almost everyone (except NE) evolves into relative outsiders
(Who can still be destroyed permanently, but hey.)
And there were equivalent of Wall of Faithless, but they scrapped it because SJW atheists were butthurt. Of course.
Most D&D settings splutter after a few releases and turn to crap with new editions. As far as I'm concerned the metaplot in Dark Sun doesn't exist beyond the death of Kalak (which is my go-to starting point for campaigns), maybe the war with Urik, and I ignore most of the material beyond the OG box and the first few supplements, like Dune Trader, Elves of Athas and such.
I shutter to think what they'll do to Planescape or Spelljammer if/when the get around rebooting them for 5e.
Though let's be honest, most of them were unholy kitchensinks from very beginning.