Hmmm. Nah. See. It's a problem with consistency.
The game makes a point of emphasizing that Aradin and his crew were in the area to try to find the Nightsong by reaching the Underdark in that area. The only way down is the temple.
In the temple and the Selune Outpost, the game emphasizes that the clerics had to seal the temple to prevent the Drow from reaching the surface because, it was the only way up.
Halsin says that Ketheric couldn't use his tower at Grymforge because the way was sealed up by the Selunites. So the only other way was via the cursed lands.
The game emphasizes that you can't leave the area by boat, because they were all destroyed. You can't go east because the pass was collapsed. You can't take the road because the bridge was destroyed by the dragon. The only ways that you can literally go are Underdark or Mountain Pass.
But this is contradicted by the Tieflings leaving for Baldur's regardless as if they magically have another way to get there that we don't. Same with the Flaming Fist. If there's another way, why aren't we presented with it? They go out of their way to tell us we can't go those ways for obvious reasons only to let NPCs go those ways.
And the same for the Underdark. They make it a point to say the only way to go is across the lake, but everyone else has seemingly lots of ways In and out. Mysterious ways.
And decrepit literally means elderly and infirm or worn out or ruined because of age or neglect.
So how is it both decrepit and recently inhabited by gnomes at the same time?
The Underdark extends over a huge area. In fact, there are countless entrances to the higher caves, not all of them by far being well known.
Another thing is the difference between the entrance through which several people (or smaller creatures) are able to get through, and the passage large enough for an army, or at least a large squad, to pass through it.
The collapsed road is most likely the road to Elturel.
The fact that the bridge has collapsed does not mean that this is the only road to BG, it is quite likely that there are other paths, although less traveled.
Remember that the area in which we can move is limited.
We know, for example, that there is a fishing village in the area, or at least its remnants.
Just because we can't go somewhere doesn't mean that place doesn't exist in the world.
Another thing is that we do not care about the road to baldurs gate at all. We have a different goal. Our team's goal is to get to Moonrise at all costs, which at the moment is surrounded by a dark curse (most likely Shar's work). It is extremely unlikely that anyone who has no choice would ever come close to this place, and even less likely that they would have survived.
Due to the fact that the curse is more than 100 years old, it also means that all traveled roads leading to the city certainly avoid Moonrise in a very wide arc.
