When people talk about saturation, declining sales, and predicting the Switch will not hit 150, etc. Most of the arguments are based on past console/handheld sales. But that also means that the market (console/handheld) has not grown at all since the PS2/DS era and that the people buying a switch are the same people for the most part who bought a PS2/DS. If the market has not grown in 20ish years then eventually it will shrink.

The Switch isn't selling another 40 million. Nintendo have had to adjust their FY projections down because sales are slowing.

A conservative decline would see it sell another 14 million this calendar year, so 134 million by 2024, and then it will likely sell well below 10 million next year.

There's not much left software wise to boost hardware numbers at this point; the system's 4th Kirby game or 6th Pokemon game aren't going to win over many who don't have one already.

The Switch has sold magnificently, but no system can last forever and we're well and truly into the waning years now; it will be six years old next week after all, almost every console ever was winding down or already gone by this point. For perspective, the Switch is now as old as the Wii was in November 2012, and the 3DS was in February 2017.

There's no shame in retiring after a great run.

I feel like the days of price cuts like this are probably over, especially in the current economic climate. PS and Xbox are adjusting their prices UP rather than down, and Nintendo have prioritized profit over price cuts for the Switch's entire life.

The video game market has grown massively since the PS2/DS era in terms of $$$ made. But Game consoles and software/services are much more expensive.

Switch at $249-$349, $60-$70 game, NSO is great deal of money made, but units wise it might not beat PS2/DS.

Mario movie could give a boost though, who knows. 

Nintendo's new management I think studied what Sony did with the PS4 which was extremely high profit margins, limited price cuts, and emphasis on digital + online service sales for fatter profit margins. 

This is how they're able to make as much money now or more than they were with both the Wii and DS together in their peak, that can't be done unless you emphasize your profit margins. Because even at 140 million+ Switch systems, that is still dwarfed by 250 million Wii + DS systems and almost 2 billion software sales between the two systems combined (Switch is not even at 1 billion yet at 6 years old, 2 billion software isn't happening). But Wii/DS did not have the $50-$60 games with no price cuts on Nintendo software, NSO monthly profit, and they have refused to budge on the Switch price point other than giving budget/kids market the Switch Lite model. 

I've seen PS2 sales data being told as being 155M, 157 or even 158M. So what's the deal? What is its real number sales?

https://gamrconnect.vgchartz.com/thread/245564/ps2-shipments-over-1586m-breaking-down-missing-fiscal-data/

Here is a thread discussing that topic. But basically 158m+ probably 159m.