Monster Hunter will never sell 5M copies outside of Japan. Unless it is radically reworked into something that is not Monster Hunter. I have played Freedom 2, Freedom Unite, and Tri. All of them are too slow paced, and too complicated to be popular with the average western gamer. Monster Hunter games require a lot of patience and practice.
If you look at the big sellers in the west, they are all games you can pick up and play almost instantly. It takes 5 minutes to figure out how to play Call of Duty or New Super Mario Bros. You have to invest an hour or two into Monster Hunter just to get a grasp on the basics.
If Monster Hunter was going to break out in the west, it would have with Tri. It had a decent push from Nintendo, was released on a console with a gigantic userbase, and had a newly-released pack-in controller. Based on the numbers on Capcom's platinum sellers web page, it looks like the game shipped 600-700k outside Japan. Not bad at all, but nothing amazing.
People mention that Monster Hunter's true appeal is local co-op, but teenagers and adults in the west don't really play handhelds in public. Well over 90% of the adults I see playing games in public are playing them on an iPhone, Android or similar mobile device. If these people own PSPs and DSs (and some of them surely do), they are playing them at home. Online multiplayer is the better option for Monster Hunter in the west and Tri already had that.