1. Retro has been gradually beefing up their staff for the last year+. Nintendo even said that they want Retro to have the ability to be working on more than one game at a time from here on out.
2. Monolith Soft wasn't a very big developer to begin with, so increasing their staff makes sense considering the scope of games like X. Plus, again, I'm sure Nintendo wants them working on something else ASAP as well.
3. Nintendo has, despite appearances, been making moves to beef up their own internal development staff as well as increase the overall amount of 1st and 2nd party/3rd party exclusives being made for their systems, for some time. They know Wii U needs software, and they also realize they need to do everything they can themselves to get more games on their console since third parties are dragging their feet.
All in all, while I would still say it wouldn't hurt them to add some company like Platinum or Vanillaware, Nintendo has a pretty strong roster of development houses. Beyond their own internal teams, Intelligent Systems, Game Freak, Arika, Camelot (sometimes), Treasure (sometimes), Good-Feel, Next Level, Retro Studios, Alpha Dream, Nd Cube, Level-5, 1-Up Studio, skip LTD, HAL Laboratory, Monster Games, etc.
Personally, I just hope to see Retro finally make an original IP of their own, just because I'm curious to see what they'd do. They don't need to just be the DK/Metroid Prime team, even though that seems to be all anyone wants/expects from them.