I didn't have a chance to play the demo, but it looks like a lot of fun.
and I'm glad Nintendo finally made a new IP for once.
wat
If you count second parties like Intelligent Systems, which is basically just Nintendo anyway, then there have been loads of new Nintendo IPs since 2000:
WarioWare
Mario & Luigi
Wii Series
Mario vs. Donkey Kong
Pikmin
Luigi's Mansion
Rhythm Heaven
Steel Diver
Chibi Robo
Eternal Darkness
Captain Rainbow
Golden Sun
Electroplankton
Battalion Wars
Xenoblade Chronicles
Legend of Starfy
Endless Ocean
Disaster: Day of Crisis
And then there's stuff like The Last Story which was made by Mistwalker (a 3rd party) but funded entirely by Nintendo (and there are even character trophies from the game in SSB4). Things like these are hard to tell who actually owns the IP so I didn't count them. Of course, not all of those games I listed are great and several of them use already existing characters, but it's silly to say Nintendo never makes new IPs.