That's simply not possible, 51 millions in 23 months?
I agree with you that it's nigh impossible, but lets just have some fun for a bit and fantasize what it would look like. Currently, Switch is at 50 million. Let's say this year is higher than last year, as many expect it to be. Last year was just over 20 million. Let's say it goes up 20%. It's currently tracking a bit better than that, but we'll assume that's just leftover holiday momentum and that a 20% growth year over year is more realistic. That's 24 million, almost halfway there, with a year to go. For his holiday 2021 scenario to happen, 2020 obviously can't be the peak year. The peak would have to happen in 2021. For that to happen, while I'm sure many would disagree and insist that Switch could just do it on its own, I personally think a hardware revision would be necessary, a Switch Pro. Lets say a Switch Pro releases this year, and next year gets an exclusive Mario Kart 9. Coupled with price cuts for the other hardware, a $300 Switch Pro that approaches the level of the original PS4, I think that could keep the 3rd party ports coming and the momentum growing, pushing sales to a final peak of 27 million, passing Wii in holiday 2021. It would take all of the above and a perfect storm of the market liking all of it, but maybe it could happen, and that's what I think it would look like.
Even with state-of-the-art hardware, a 5000 mAh battery wouldn't survive more than an hour, if it even runs that long. Remember that what turned most people off before the new model came out was the lack of battery life, and with such short life, nobody would ever buy it. Also, this would need a bigger cooler, which would make the Switch much heavier and harder to take around with you and play in handheld mode.
Something this powerful may become viable around 2023, but by then, this could just as well end up as a successor instead of a Pro model.
Also, the report that there's a Pro model incoming with Volta GPU part sounds very dubious to me, as Xavier is probably too big and axed towards scientific calculations and AI, not consumer graphics. I think the leaker saw the codenames of the upcoming Xavier Models, saw that is NX (Like the one from the Switch; but it's also short for NVidia Xavier...) and thought that that was coming for Switch Pro.