We have an official response about this:
http://www.polygon.com/2017/1/19/14313914/legend-of-zelda-breath-of-the-wild-last-nintendo-game-for-wii-u
so, the servers won't be closed with the launch of the Switch, it seems that activity is an important factor to decide whether or not keeping the servers working.
http://www.polygon.com/2017/1/19/14313914/legend-of-zelda-breath-of-the-wild-last-nintendo-game-for-wii-u
so, the servers won't be closed with the launch of the Switch, it seems that activity is an important factor to decide whether or not keeping the servers working.