That's a cool outline, but my problem with it is that fundamentally it relies on meta knowledge. Why would the characters go into the underdark unprompted? Especially if you go down via the spider's lair, there's nothing suggesting you'll find a way back up. Nothing even suggests that there's anything of note down there. Why would there be? It's a random hole in an underground cavern that actually has several other random holes in it. Say what you want about the reliability of the leads we have on the surface, at least they're in relatively close proximity for investigation. The Underdark is supposed to be HUGE. Even if you did think you could find stuff out down there, there's no reason to think you're not just falling into the equivalent of a massive, unpopulated prairie.
This is a big issue I have with BG3. People talk about all the options the game gives, but most of them only exist if you metagame and already know about them.
I am not in agreement that it relies on meta knowledge inherently, or to put it another way an outline is impossible without meta knowledge. So the point is irrelevant. Don't conflate the outline with the way the game is played or the clues that would lead an intelligent, power-hungry player into the Underdark - a place known to be full of powerful and dangerous creatures, but also fabulous wealth and resources.
As for getting out, are we not aware of the jump points that allow us to instantly teleport to previous locations? Are you planning to argue that "we don't know if they would work or not?"
Additionally there are personality subtypes that emphasize high risk/reward behaviors and those subtypes tend to fall into sociopathy. Arrogance goes hand in hand with a lust for power, and while some caution and knowledge gathering would be warranted - a certain amount of danger would not prevent risky actions. You do realize we all just killed a massive spider creature after all? We are clearly a cut above the mere peasantry.
I would invite you to consider that you are not used to roleplaying evil, arrogance or power-hungry individuals and so that is not a skin you are comfortable in or familiar with. The "good" mentality is a community focused one, and tends to be more risk-averse when presented with unsanctioned paths/behaviors. They tend to be more comfortable with Risk when its sanctioned by good authority figures and presented within the ur-narrative that portrays them as 'heroic'.
