I drive on the right side of the road in the United States, so it makes absolutely no sense that I can't drive on the right side of the road in England.
If driving on the right side of the road caused innocent people to suffer for eternity - even if it was a minority of people - don't you think there'd be people who objected to right-side-road driving?
Even if all the lawmakers decreed that it was necessary because otherwise there would be chaos, don't you think it'd be important - even
necessary - for those who cared about the suffering of others to
do something about that law? Or should they accept it as the way things must be because it is the way things are?
After all,
someone put that law in place. It was chosen, perhaps at the creation of our body of laws, perhaps by one venal and wicked lawmaker, but other choices are possible. I mean, England clearly made another choice. Some of our lawmakers are even originally
from these other places, many of which
don't require that you cause eternal human suffering on your way to work in the morning.
That's FR's situation, using your analogy.
That argument doesn't hold up because they are completely different locations with completely different laws and rules. The reasons for those are many, nuanced, and varied, but it doesn't do you a whole lot of good to cite American law in an English court or vice versa.
But you can change law. Laws, like FR's soul arrangement, were
put in place by powerful people, and if there is reason to change it, they can and do change. And the reason is that wailing, moaning, suffering bit of mortar that yesterday was a dragonborn paladin who was saving orphans from being drowned by night hags.
Forgotten Realms and Greyhawk aren't even in the same universe; they are on completely different planes of existence. Expecting them to have the same rules seems like a bit of a stretch to me.
As the setting has been written, that's not really true. FR is home to LOTS of links to other worlds - many of the gods are even from other worlds, Greyhawk among them. It's more like Toril is one planet among many in the same cosmos, with Greyhawk being another one, and there's also gates that link them together and wizards can travel between them and spelljammers can flow in the space that separates them. This doesn't even involve going to other settings - FR does plenty of this in things with FR's own name on 'em.
But even if it WAS true, there's every reason to suspect that the current set up of souls is a scenario that can and should change.
When I compare D&D campaign settings that I'm familiar with, there really isn't any sort of consistency to the afterlife. Greyhawk follows the Great Wheel model where souls go to their alignment planes. Forgotten Realms is a modified version where all souls go to the fugue plane first. Dark Sun has everyone go to the Grey with NO hope of going past that (and no gods). Eberron has Dolurrh which seems about the same as the Grey, except that numerous religions preach that your soul will be joined with your deity in the after-afterlife, though there is no tangible evidence of that. (If anything, Greyhawk stands out as not having a crappy, depressing place as the first stop of your soul).
FR, according to its own material, shares a cosmos with Greyhawk (or "D&D," in 5e). Dark Sun, likewise (there's githyanki and planar travel and different Ages and whatnot). Eberron...not so much.
In short, I think there are plenty of arguments for allowing the Forgotten Realms to stand alone without holding it up to the standard of Greyhawk or Planescape. The more compelling argument to me are the different continents on Toril that apparently have different afterlives. That, to me, is probably a result of trying to force different campaign settings onto a single world, which is really just poor design choice.
Whether you agree with it or not, it's how FR presents itself. If you're talking about FR as a setting and not as you might play it, part of that baggage is that it is one of many worlds in a cosmos.
What I have been arguing against is the concept that there is no justification for the metaphysics of the Forgotten Realms and that it is inherently wrong. A lot of the arguments in this thread have seemed less "I should be able to play an atheist character" and more "playing a good, devout character is impossible". As such, I have tried to present numerous explanations and justifications for how the metaphysics can be interpreted in such a way that allows for good and devout characters.
And, as I said, I find statements of "that is wrong" to be boring. It is far more interesting to take seemingly incongruous facts and try to make a cohesive whole out of it.
Fact: The Good gods are good.
Fact: Faithless are tormented and put into the Wall.
Easy and boring: That is wrong.
Complex, interesting, and entertaining: How can I make this work?
One of the ways you can make it work is by having the Good gods conspire to kill Kelemvor and tear down thew Wall, knowing that it is wicked, possibly going against Ao in the process.
That makes a more interesting story than "This is just the way it works, accept it!" does.