Did I not make myself clear enough?
First off, even in your ideal world where every group takes only roughly 10 minutes to make, that doesn't mean every single dungeon you're getting a piece of gear anyway.
You are going to be spending hours upon hours grinding them out just to get upgrades that will trickle in outside of the vault, which is a weekly event.
Again, this whole weird mindset only seems to show up on forums, where people think that +15s are "easy" and "faceroll" whereas in the actual game, a vast majority of players struggle with them.