I think the success of the Switch will decide for them. If Nintendo can remain in the hardware business with moderate success then there's no reason for them to go 3rd party other than to benefit PS4 and XB1 owners.
I believe the Switch is a step in the wrong direction though, and I wouldn't be surprised if in a few years time going 3rd party seems like the better option. Whether they will though is another question, a Nintendo without hardware is more likely to fade into irrelevancy.