Yes, flamebait. The OP is basically presenting an argument that is as credible as Donald Trump's claim that global warming doesn't really exist and is an invention of China.
Any success that can be attributed to the 3DS is in spite of 3D. There's the early price cut that resulted in Nintendo posting their first fiscal year loss since they had entered the video game business. There's the 2DS which was not planned and only exists because the market's rejection of 3D was so strong that Nintendo became convinced that such a desperate move is necessary to stabilize the platform. There's the New 2DS XL which gave consumers finally a fair choice between 3DS and 2DS, because the initial 2DS did not have a clamshell design; and what do we see consumers choosing? They buy the New 2DS XL in higher numbers than the 3DS models now.
And finally, there's Switch. If 3D was a success like the OP wants you to believe, then why is it non-existent in Switch? If 3D was a selling point, then it should have become standard and Switch should be floundering for not having it; instead Switch flies off the shelves, showing how unnecessary and unwanted 3D was. Switch also casts major doubt on the belief that mobile was killing handheld gaming. I never believed it because touchscreen inputs are so limited that they in turn greatly limit the variety of games. But those who did believe it and probably still believe it... they should really ask themselves if it isn't more probable that self-inflicted damage by Nintendo was a much bigger cause of the decline from DS to 3DS.
As for the Virtual Boy, the story is that Yokoi was opposed to releasing it in the state that it was. I've touched on this in my first post in this thread.
A 3D slider would actually be a nice feature on the Switch in handheld play. But, it would have pushed the device past a $299 price point, which would have been very bad for business. And ALL of the software would have to be designed as compatible with that feature. That wasn't going to happen.
A fair amount of the 153 million people who bought a DS prefer those "limited" games that can be found in the mobile market. Unless the Nintendo Switch sells over 150 million units, the mobile market did take that chunk of people away from handheld, and they aren't coming back.