Well, I'm not one to tell them to instantly go third part, but:
You can start with the fact that they ended up having a loss in the Wii U department regardless of strong software sales [http://www.polygon.com/2014/1/29/5356772/nintendo-falls-short-of-hardware-and-software-sales-forecasts]. Essentially, their games are popular, but the console is hurting them, and the games' popularity is being squandered due to being on an unpopular console. Now, this wouldn't be as problematic if they actually released a good console with decent third-party support, but it still shows that their hardware department is sort of dragging them down right now. Even the Nintendo 3DS couldn't pick up the slack from the Wii U.
Now, going third party because of this is a short-term decision that would likely hurt them compared to what they could do. What Nintendo should focus on is actually delivering a competitive console and fostering better third-party relationships. People want to play Nintendo, but they don't want to buy the system, so they ask for Nintendo to go third party. That's certainly better than Nintendo's current status, but they would likely be served better by actually fixing their model so that they make consoles gamers want.