Nintendo should stay 1st party because once they stop making consoles they'll also start abandoning the console industry as a whole. Their portable division would be moved to mobiles immediately because they'd never choose the Vita over mobile platforms or a portable system of their own. They'd probably make a few games for home consoles but sooner or later those efforts would be moved to mobiles as well.
Without a system of their own Nintendo would not support games such as Wonderful 101, Bayonetta or Devil's Third. No need to waste money on games that won't bring good profits. Expect even more Mario because money.
Believing that Nintendo would become a huge supporter of PS4 and Xbone is nice and all but Nintendo taking that route is about as likely as them turning the Wii U situation around and having it become a 100m seller. Pursuing the old Wii/DS audiences would be a much more attractive alternative.
I want them to stay in the console business and they'll only have a good reason for doing so while they have systems of their own to support, thus I want them to stay first party.
