we mustn't look into past for find new solution but really in the present.
Good game design never gets old. People still play chess nowadays and it isn't the youngest game.

The issues you re mentioning don't arise necessarily from open world dungeons, but rather from sloppy rewards system mechanics, which can be addressed separately.

It's a necessary evil sadly to keep dungeons simplistic cause the moment you create options, you only create obstacles that make the DF process harder.
When you have to design the content around the DF, then you have a huge game design problem. The Df is a tool to help players find groups, it shouldn't dictate how the content is to be designed.

No ones asking for an entire world in a dungeon.
Why not ? That would be great. Especially if you wouldn't be able to do the whole dungeon in a single run (it would add a replay value to the dungeon).