Highlighting a specific point doesn't mean there aren't other points besides what you're focusing on, it just means that Nintendo wants to draw attention to the games they have coming in 2015 at E3.
Nintendo could announce a bunch of new games releasing in 2016 at E3, but they could use a bunch of short gameplay trailers or teasers for those games and give the 2015 games they're announcing and showing new details on the most time in their presentation.
The fact that Zelda may end up becoming a 2016 game should actually show people that the Wii U will continue to be supported beyond this year and the fact that Nintendo has never released a new home console less than 5 years after their preceding platform should make people understand that it's very unlikely that the next Nintendo home console will release before Mid November 2017.
They have said they hope to share details on NX next year, but that doesn't mean they'll be ready to release it in 2016, so all signs actually point to Wii U getting games support from Nintendo until at least early Fall 2017.
If NX is in fact the Fusion system then Nintendo will have bigger game output at the launch of that system, because they can release all of their console software across both the handheld and home platforms simultaneously.
NX would make it possible for Nintendo to literally treat their new handheld and home console as one, as far as marketing goes, so at E3 2016 they could still have time in their presentations to announce games for Wii U, even if they mainly focus on the new system(s).
