Will 2015 E3 be the last Wii U-focused E3 for Nintendo?

[Soundwave]

Yes.

They likely are internally starting to transition all their big teams to NX projects right now. Game development doesn't just happen in 5 months.

For a hardware launch you need to start developing software about 2 years in advance of the launch. 18 months is a normal game development cycle, though many of Nintendo's cycles go even longer than that. Factor in an additional 6-7 months for developers to get the hang of the new hardware and properly plan for utilizing the new hardware, development would need to be starting about now even for a mid/fall 2017 launch.