Board Thread:Metal Gear Solid V/@comment-5.78.74.226-20150913090927/@comment-25366375-20150913104130

The long and short of it; you've already finished the story. Those Extreme missions etc. don't advance the plot. Once you do Mission 46, the game is over from a story point of view.

There are all sorts of conspiracies and apologists saying "but wait, there might be more!" but until that's confirmed or denied, you've finished MGSV's plot.