...they'd need to come up with a plot contrivance to explain why the player suddenly needs to visit the Upper City and can't just fight the Netherbrain.
Gortash should have been in the Upper City all along instead of on a bridge.
That way the characters would have to navigate their way through Rivington, figure out the state of the city, find out what's happening with murders, then infiltrate the Lower City. In the lower city, they should have to continue the quest to solve the murders to find Orin. Additionally, they should have to discover a way into the Upper City. In the Upper City they can confront Gortash, side with him or not, and they can visit Lorroakan and Cazador.
I'd argue it's not a contrivance. It's probably the way the game was originally designed because it just makes more sense, simply put. The "contrivance" is what we have now.
