Oh I agree. But that's not what they did. We got 7 classic adventures and an introduction that explains the tavern and the well. It even talks about the patrons betting on the groups who venture down. Seems to me they would want to make money on the dungeon, so they'll have to release another...
I wouldn't call it a spoiler since the info is already out there, but the yawning portal is a tavern in Waterdeep with a well in the center that is an entrance to Undermountain. But Undermountain's not in this book. That dungeon could easily serve as its own book. And Dust is one of two 50...