I don't think we've lost the dungeon, I think the dungeon has evolved from 30 years of its traditional sense. 30 years ago, the idea of going into anything and hacking up monsters as someone else was faciniating and the logic ofa dungeon full of these things near a populated town for years didn't need to make sense.
Why is this dungeon here? Lets make a city to explain... Why is this city here? Let's explain the Kingdom... Why is this Kingdom here? let's explain the world.
But as d and d evolved so did players whom wanted more explanation and control. So the world became important. The world defines the dungeons in the land and why they are there there. It gives the dungeon meaning. Dungeons have evolved from crypts, towers and underground layers. Now dungeons can be winderness adventures in which the walls are the trees, political intrigue where the minitours are the corrupt goverments, mystery's where the treasure is the answer.
I dont believe the traditional dungeon has lost its importance, however. In the end there is always something that needs to be slain and somewhere it needs to be slain in. I try to give my pcs a good mix of the different varieties of gameplay. Right now, they are actually in a traditional dungeon after a few weeks of political intrigue and wilderness lore. It has made them appreciate the dungeon more, because I spent that time giving this dungeon meaning.