Cannot agree more with this idea. We've went through the instance, which is more like a quest.
Now we should be able to have free access to an uninstanced version of the dungeon.
It doesn't make sense that we've went in and killed off the boss at the end, effectively making the place a little safer, and then we can't explore it.

The area should only be accessed via the entrance however, and would give people a reason to traverse the open world, instead of sitting in a camp/city/lobby and waiting for Duty Finder...