I wonder if hacking off pieces of act3 is the last minute crisis that Sven mentioned in the FPS podcast.
As a software engineer myself I think it is possible that there was something really nasty going on at some "critical point" in Upper City that the whole part had to be put aside for the time being. If the access to Avernus happens in Upper City, too, then this may be a reason.
I can also imagine that the missing epilogue has something to do with this as well, maybe it simply made no sense having the epilogue in there while Upper City is not part of the game.
Notice how I say "put aside for the time being" and "while".
I wouldn't rule out that patch 1 and / or 2 bring back the meaningful parts of the content that is currently missing.
