Question:
Where did those bandits came from? Is their existence conditioned by failing dialogue roll in completely different situation?
So ... it means Larian would need to prepare and script combat scenario that nobody who dont fail there will never see? O_o
Hell yes! I think the beauty of RPGs is that different things can happen and you just might not get to experience the whole thing, at least not in one play through.
Another question:
How exactly should game decide wich of those two outcomes for each scenario (fail/succes) should be used?
This is where I didn't put my thoughts into text very well. It's not that the game is deciding, there might still only be 2 outcomes, but if many times during the game the outcomes seem like in a gray area, YOU don't know if it was a success or failure. But you're apt to find out later ;-)
Another one:
Im sory but i have to ask ... what would be the point of rolling anyway?
I mean, i see that there is difference in your example ... but when its not clear if i even suceeded or failed, there is no way to find out, and all outcomes seems to be same at first sight ...
Why should i even care about what, why, or how i roll? :-/
The easiest answer for this is because it's D&D, the game we came here to play.
Would you ask your DM "what's the point of rolling if I don't know if I passed or failed?" But, maybe you would ask that. I wouldn't because the DM is weaving the story for us and I'm trusting them to give me an entertaining experience.
And finaly most important question:
Since the argument was savescuming ... does it somehow prevent it?
(Just for the record i still think it cant and shouldnt be prevented, but i try to stick to the topic.)
I mean what is holding me back to open that chest, find out its full of junk and reload anyway?
Nope, it wouldn't prevent it, but I feel like it might discourage it if the result of the effort of savescumming might not become apparent until hours later, and it might turn out you would have preferred the original option!
Imagine you decided to reload and reroll until you finally convinced them to give you the locked chest to deliver rather than politely denying your request. Then you found out that the lock can't be picked and maybe hours later in the game you get ambushed and it's a bad experience all around. Then you find out you denying your offer was them deciding they liked you and didn't want you to get ambushed because their cronies were waiting to ambush anyone coming along the road with the "chest of goodies"
Another question:
Wouldnt that also remove good feeling of succes?
I mean ... if i see that i will need to roll 18+ ... and i do ... there is kinda "yes!" feeling ...
Is it here aswell? Since i dont really i know if i suceeded? O_o
I cant help bug feel like it wouldnt be.

Maybe yes, maybe no, different strokes, different folks...