Only in Planescape. Not in Forgotten Realms. PS was laid over top of existing Realms, not the other way around. If you, like me, completely ignore all Planescape material, then the idea that a soul goes to an alignment plane never existed in Forgotten Realms. In FR, the souls go to whatever realm of their patron god, regardless of their actual alignment. As was mentioned, a LG paladin of Sune goes to a CG plane for final disposition.
Think about it this way. Forget PS canon for a moment. In FR, a soul goes to whatever god's domain that soul favoured in life. The only function of souls in Forgotten Realms is to power the gods. Whether you're a free range farmer or run a chicken factory, at the end of the day, that chicken is still going to be eaten. Thus it is in FR. Anyone born in FR goes to whatever plane it's supposed to based on who they worshipped in life.
Those that refuse to believe are basically tainted meat. They don't actually do anything for the gods. So, if there was no Wall, those souls would just wander around the Fugue plane until such time as they were eaten by a demon or tempted by a devil. They don't go to an alignment plane because souls in FR don't do that, in the FR cosmology as presented. So, if the gods tear down the Wall, all they do is spawn more demons. Those souls don't go anywhere. In FR cosmology, there's nowhere else to go. You can't go to a god's domain since you denied the gods.
The only thing messing this up is the insistence that Planescape elements somehow apply to Forgotten Realms. That the afterlife presented in PS applies to all D&D settings. But, the settings themselves don't do this. That's why we have things like The Wall in Forgotten Realms. In Dragonlance (unless this was changed later), there aren't any devils because there aren't any Hells. Only the Abyss. Souls either go to the Abyss if they were evil or to some sort of Heaven (whose name I am blanking on right now) if they were good. There's no Limbo, no Hell, only two planes in Dragonlance. PS came along and TSR tried to go back and change all that, but, that's my point. None of the settings originally used that cosmology.
The only reason that the Wall is seen as a terrible evil is if you apply the PS cosmology where aligned planes await the dead. But, that directly contradicts Forgotten Realms canon where your afterlife is entirely dependent on WHO you worshipped, not your actual alignment.