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.
Increasing the price point is only a mistake if it's due to a feature that does not qualify as a selling point. Since 3D has proven itself so clearly as a burden, its exclusion from Switch was a no-brainer. This is about the premise of your thread: Your claim is that the sales numbers of the 3DS prove that 3D was a good vision, forward-thinking or whatever other positive description you can come up with. But all of the evidence that stems from 3DS sales numbers points to 3D being a great liability. Hence why I say that the 3DS's ultimately solid lifetime sales are not because of 3D, but in spite of 3D.
Switch doesn't need to sell DS numbers to prove that "mobile killing handheld" is untrue. At 100m Switch consoles sold it will already be undeniably that Nintendo self-inflicted damage on their handheld market with the 3D feature of the 3DS. At 125m Switch consoles sold it will be undeniable that the main culprit for Nintendo's decline was Nintendo themselves. At 150m it merely means that "mobile is killing handheld" has little to no ground to stand on anymore, so the argument could be expected to simply die off.