I would disagree and say a few cross gen release games for the install of NX to increase are far more likely than Nintendo going third party. In fact, if their movies happen to do well I could see them drop out of video games all together sooner than I can see them going third party, and focus on other media. Hopefully we won't have to find out.

There's no point continuing to bring games to Wii U once NX comes out, it's a dying platform, and by then it will be dead. If NX doesn't work out, they can't just fall back to Wii U, just like they can't just fall back to the Wii now that the Wii U has failed.

It's not a permanent fix, but a temporary one they can use to re-evaluate their situation. That's the beauty of the first year when people are still making the switch. Big companies cross gen games are common place due to selling more than just what they could on a new platform. It's nothing new.

Nintendo didn't release many cross-gen games across Wii and Wii U. Or, TP aside, on Wii and Gamecube. Or Gamecube and N64. Or N64 and SNES. Or SNES and NES. 

Look at at this way; if NX comes out next March, Nintendo are going to want to give it at least one holiday season before giving up on it, since that's traditionally where a massive proportion of their sales come from. So the soonest they'll likely be giving up on it is January 2018. By then, practically everyone will have moved on from Wii U and migrated to other platforms. Their Wii Us will be either sold on or gathering dust in a closet, while they're too busy playing the abundance of games on Playstation/Xbox/PC to be bothered rebuying/digging out their Wii U.