I chose never. But, as I always say when people talk about lifetime sales of current hardware, the question doesn't really work anymore. Generations are changing. They may not exist in the way that we've come to know them. If every system is backwards and forwards compatible with the 1-3 that come before or after it, as seems likely to be the case, where does a generation start or end?

2019.

2 years ago I would've said never but then Xbox didn't put up a fight whatsoever. Now I'd say it's possible before 2020.

Wanna bet on that?

If nintnedo converts the ds fans the Xbox one is screwed. This starts with petitioning all of the third party who worked on the ds to take the risk and make spritual successors to their ds games on the switch. If nintnedo has a mainline Pokémon game and throws all of their high quality first party at their new console/handheld... yeah, I can see the switch passing the Xbox one.

I voted for "never".

Xone is at 30 millions or something now, and will be at 45 millions (or close) near the end of 2019.
Switch will be at 13 millions after march 2018, you can double this number if you're optimist. It's still only 26 millions for march 2019.

I don't expect the switch to get a long life, 2021/2022 at best.

And unlike 3DS, nintendo can't really make tons of models with the nintendo switch, we will probably never get a swich XL, or a new switch.

There doesnt need to be a Switch XL. Nintendo just needs to pull off the proper marketing scheme, have a healthy library and have yearly stability with third party and they should be fine. No offense to Microsoft, but there is no reason to underestumate the potential of wooing the Nintendo handheld crowd. A mainline Pokemon game can bring them over to the switch. 

The Switch will never past the Xbox One since all of MS next gen console will belong to the Xbox One family

14m this year, 20m next year.
The flash shortage will end in 2018, Nintendo will be able to cut the price then.
So I think the Switch will outsell Xbox One as of now in 2nd half 2018.
It will outpace Xbox One by end of 1st half 2019.

• Bring out a model without active cooling and over 2x battery life in a smaller body and call it the Switch Slim.
• Bring out a more powerful model, same size as now and call it the Super Switch, 1080p+ OLED screen, better kickstand, better wifi, docked performance (current Switch) as undocked experience.
• Make a Switch Lite, with a smaller body, bezels same size screen (or move to a 5.x" screen) take out the active cooling and fuse the Joycon into the body, make a smaller dock.
• Switch XL increase the size of the screen but remove the bezels so the Switch XL does not take up any extra space, and you keep Joycon compatibility.
• Switch Go a super small device built for kids (2DS) Joycons fused to body and the screen is downsized to about 3DS XL level.