I agree for the most part. Just the part about Japan's sales doesn't really seem all that justified considering how frontloaded the DS was there. Switch should be catching up starting this year as the sales came down quite a bit in it's 4th year while the Switch is still climbing, coronavirus notwithstanding. If it can fully catch up there, that's another question, but the gap should start to shrink this year. Same goes for your comparison with the 3DS in Japan, which the Switch is close to guaranteed by now to overtake considering how the 3DS dropped off in his 4th year, too. I do expect Switch to land around 27-32M LT sales in Japan, which wouldn't be far behind the DS's total.
Just one thing that could play in the Switch's favor though that you possibly missed or just forgot to mention: The fact that console generations are getting longer. It could result into the Switch having a longer shelf life than the DS did, meaning it could creep up on the DS to the end of it's career when DS sales were already very low. DS had 6 very good years but dropped off like a rock afterwards; if the Switch could sell for, say, 7 good years (15M+ each year until 2023 included) and a slower decline afterwards, it could do it.
But like you said, chances are extremely slim. Without a shelf life similar to that of the Gameboy or the NES, I don't think Switch could do it.
Console generations got slightly longer for PlayStation & Xbox. Going by U.S. release dates, the PS2 was released five years after the PS2, the PS3 was released six years after the PS2, the PS4 was released seven years after the PS3, and the PS5 is scheduled for release seven years after the PS4. The 360 was released four years after the Xbox, the XBO was released eight years after the 360 (the longest gap between home consoles among the Big Four), and the XSX is scheduled for release seven years after the XBO.
With Nintendo, the longest gap for their home consoles has been six years, which was between the Wii and Wii U, with five being the average. Aside from the anomalously long lifespan of the Game Boy (~12 years between it and the GBA), the gaps were more or less consistent with what we see with home consoles, with a short 3-½ year gap between the GBA and DS, a nearly 6-½ year gap between the DS and 3DS, and (if you want to count it) a six-year gap between the 3DS and Switch.
Realistically, we ought to not expect the Switch's successor any later than early 2024, probably 2023 most likely. That gives it another 3-4 years of primary life. Even if we grant it another five years, to beat the DS the Switch would still have to have implausibly high growth over the next couple of years. The Switch's current sales levels and what growth we've seen out of it simply don't seem very encouraging for it to be able to sell the staggering quantity it would need to sell to close the gap with the DS. Barring some significant sales growth this year, I don't see it doing much more than 120M globally, and that's what I'd consider optimistic.