It comes down to expectations versus reality. Many of us gamers are pretty simple in what we want, a steady evolution of tech and games. Nintendo has made a lot of decisions in its history that went against the grain, and because of this many ideas are predicted as failure for not conform to conventional thinking.
Think about this, what will the PlayStation 5 look like? Probably have a good expectation and think about processor, hard drive, RAM, even down to the controller and shape of the system.
Now, what will Nintendo's Switch successor look like? I have no clue.
Coupled with this was the Wii was a cultural phenomenon the way a gaming system hasn't since. So many people can claim it as their only dedicated gaming system. This mix of not catering to forum gamers and being a huge success earned it a place of being a threat. Imagine if gaming changed and Sony and Microsoft and gone full on "Wii Mode" and not just add-ons like Move or Kinect, but the Xbox One and PlayStation 4 were barely more powerful than their predecessors and emphasized motion controls.
Gamers are use to being catered to as they were the bedrock of the industry for decades, to think they could be disposable and see their passion change into a general hobby of the masses scares a lot of them.
