You got to realize is that Nintendo is like Universal Studios. They may be the largest, but putting out consistently high quality game after game, year after year is something that is unfeasible for one company to do. There are going to be some slow times when projects just aren't finished and yes, they can probably see them coming a mile away. So how would you fix this situation? Buy more companies and become a bigger conglomerate? I'm sure they are trying rigorously, but it is easier said than done. Just like with all business, a relationship needs to be built first, both companies need to get what they want, and the outcome needs to be at least or very near Nintendo's other development teams.
So what has Nintendo been doing? Both at once. They allow teams they are interested in to develop ports or lesser IP's to see how well they do, upgrading them to bigger or very different projects.
There is only one problem. Nintendo can't do it all of course. No one can. So not to dwell on third parties, but they play a crucial part to diversity and numbers as well.
To make matters worse, this is always moving, always changing and instances are never the same. They may have milked ports in the past, and as a company, they have the right to if it makes business sense, but again, never one situation is the same.