Old thread dug up, interesting.
The PS2 will remain on its throne. As will the DS. The best any platform (even the Switch) can hope for is third place. If I'm being more generous to Nintendo, Switch could possibly sell 140-145 million units.
Switch 2 isn't coming out really soon, I would think. March 2023 at the earliest, and it will probably be after that. Once that happens, the Switch can't logically sell 30 million or more once its replacement is out. PS2 sold about 40 million units after the PS3 launched. That won't happen with the Switch. The Switch 2 launch will not be as much of a blunder as the PS3.

Ps2 came out less than 5.5 years after PS1.
Ps3 came out over 6.5 years after PS2.

This gave ps2 an unusually long period for success and support. Combine that with the fact that PS3 wasn't very popular at launch due to price and it gave PS2 even more legs.

People celebrate the PS2 like it achieved this great thing but in reality it smothered its own parent to death slightly ahead of schedule and was stepping on it its offspring to maintain it's position. Did it sell great on its own? Yes. But it wouldn't be number 1 all time if not for the reasons above.

I believe the new king will be the Switch

Very true. But that's why it's hard to dethrone. Switch released after a failed console (and handheld that fell below expectations), and will be succeeded by a (almost guaranteed) successful one. 

The Switch 2 will take some sales away from the Switch for sure if Switch 2 releases in 2023 or 2024 (very likely). 

PS2 was in some special circumstances, I totally agree that's why it was king. It's also why PS4 has done so darn well besides playing it safer than Xbox One, Wii U/Switch. Obviously PS4 won't touch PS2's total, but PS4 was also shaped by many circumstances. 

If Nintendo give Switch 10 years then?

10 years would mean it was still selling well in year 7-9, which would mean it would have probably already achieved DS/PS2 levels by that point, and then shattered those records by year 10.

3ds still get new games in eshop and still sell a bit each week, Switch definitely have a chance.

Does anyone know when (month and year) the PS2 officially claimed the title of "best selling console of all time"? It will be interesting to see how long it was able to keep the title when the Switch finally knocks it off of its throne.

I mean in 2006-07 I rather get a console at $80 with a massive wall of cheap used games to choose from at Gamestop than a $600 system with almost nothing to play. (I know the MSRP was more than 80 but we sold it at 80 and sold a lot of them)

As well it literally had no competition in the home console market.