What's the PS2 have to do with this?
In any case, we can look at older systems to look at trends to see when a system experiencing a period of growth might pass its peak and enter its decline phase (note: the N64 and 3DS were omitted from these charts because they peaked within their first year).
Except for the GameCube (for reasons I'll explain in a bit), every Nintendo system had a period of at least a few quarters where they experienced year-over-year growth (with the DS it was 14 straight quarters of YoY growth, the best such streak for any system ever), but that growth slowed and eventually flipped over into YoY declines. At no point in any Nintendo system released in the past 20 years have we seen a Nintendo system experience a period of growth, then some declines, and then another multi-quarter period of significant growth. It just doesn't happen. Once we see two or three quarters of YoY declines, that has always heralded the start of the terminal decline phase of a Nintendo system's life.
The GameCube was an odd duck because it had a price cut very early that gave a good boost to Q2 sales, but that had petered out, hence the YoY declines in Q2 & Q3 2003. Then it had another price cut in Sept. 2003 that gave Q4 that year a big boost, but that boost was even shorter-lived that the boost caused by the price cut.
However, we do know the following two facts:
1) The Switch's YoY increases have been declining and, regardless of the reasons, Nintendo systems experiencing a multi-quarter growth period see said growth slow and then flip over into YoY declines, which initiates the post-peak period of the system's life.
2) The Switch's YoY increases have not been of the same magnitude of growth that the DS experienced. It's not even close. They're not even in the same league as the Wii's YoY increases during its growth period. They're more in line with those of the GBA.
Based on these facts, I simply do not see a multi-year increase in Switch sales in the cards (there's also no precedent for a multi-year period of flat-sales for Nintendo systems). And without rapid growth over the next two years, that gap between the Switch and the DS will grow and fast. Is it possible that we could see continued and significant growth out of the Switch? Yes. Is it likely? No. Could it potentially meet or beat the DS? Not a chance.
And as for the total size of the market, that's such a nebulous quantity as to be useless. After three generations, we can surmise that the total combined global market for PlayStation & Xbox is in the 170-180M range, something that's been pretty damn consistent so far (even the final WW total for combined PS4+XBO sales will likely end up in that range). But when it comes to Nintendo, they've been highly volatile. They went from the GameCube (bad sales) to the Wii (good sales) to the Wii U (bad sales) and now to the Switch (good sales). Even their handhelds have not shown much consistency. And this is all before taking into account regional differences in sales. And counting the home console and old handheld market separately is even less useful. We have only a vague idea what the overlap between Nintendo home console owners and PS/Xbox owners are, and we have even less of an idea what the overlap between what, say, Wii and DS owners were, or DS owners and PS/Xbox owners.
We have no idea that Nintendo's total potential market size is, and it really doesn't matter. The only thing that matters is what the sales data says, and as more time passes we'll start to get an ever more clear picture of what the Switch's trajectory will be.
Here is an article on the year-on-year sales of the Switch. I calculated it and in 2019 YoY covering the same time period, Switch's sales grew 17%. In 2020 however Switch's sales grew 21% YoY over the same time period despite having a weak Jan-Feb so far, which shows that the Switch is still growing globally in sales.