Switch could beat it ,but to early to tell.
155mill units is insane.

If it sold 30 million this year and next year that would put it at around 140 million by the end of 2022. Sales would have to drop to less than 18 million for the rest of its life to not outsell the ps2. 

Now I'm not sure if the switch will outsell the ps2 or not, but in the hypothetical that you provided it definitely would. There is no chance that if the switch sold 30 million in 2022 that it would be replaced in 2023, so that would guarantee at least 20 million in 2023 which alone would put the switch at around 160 million.

The real question in my mind is how long before Nintendo launches the successor? That will determine whether or not the Switch can do it. Nintendo has said they want a 'long console life span' for the Switch and that could be defined as anything from 7-10 years, although if the latter I'd expect the mid-gen upgrade to be a necessity. Regardless, it's currently at 80 million four years in and is trending ahead of last year. It's hard to say whether last year or this year will be the peak sales year, but I figure that either way I feel they will be relatively close.

For the sake of argument, let's say 2020 was peak sales year and this year it sells slightly less: 25 million. That would put the Switch at 105 million with 55 left to go. If Nintendo only gives 2 more years (Switch 2 in March 2024, 7 year cycle) it would require 27.5 million average per year and of course won't beat PS2. If they go for 3 years (Switch 2 in 3/2025, 8 year cycle) it would require 18.3 million average per year, which is possible, but unlikely.

Now, I haven't been including after-successor sales, because I think they will be rather low. Whenever Switch 2 comes along I assume it will have a similar hybrid model and thus be backwards compatible to Switch games. It would have it's own slate of exclusives, and after the long lifespan of the Switch I don't think that system will have much after-market success. What we are currently seeing with the PS4 and Xbox One seem likely to be the new paradigm.

Then we get into the long console lifespans. If Nintendo launches a mid-gen upgrade sometime within the next year or two they can potentially push the Switch to a 9 year (Switch 2 in 3/26, needs 13.5 million average to surpass PS2) or even 10 year (Switch 2 in 3/27, needs 11 million average to surpass) lifespan. In either of these situations, and especially the latter, I believe the Switch could do it. The refresh could make up for aged hardware, preventing it from being too dated and continued refinement in dynamic resolution and games built ground up from the Switch could still look good. We've hit a point of diminishing returns on graphic power and you can make the argument that less graphically intense games on the Switch can look better than cutting edge tech with strong art direction. For example, I'd take Mariokart 8 Deluxe over several modern games that focus on hyperrealism.

The question also comes to what is the saturation point? I don't think it will be a low one for several reasons. Primarily because I often see households with multiple Switchs, whether it's multiple children having them or even both halves of a married couple. From my own experience people tend to treat it as a personal device and buy multiple rather than share like you would for a standard home console. Add to this the expansion of the Asian market. Taiwan, South Korea, and even China are buying more Nintendo consoles than is typical. Nintendo's deal with Tencent let's them have an official release in China which while I don't expect to be massive, will contribute to pushing the saturation point higher and higher and Korea/Taiwan have become enamored with ACNH and Ring Fit Adventure, the latter of which is an experience that requires a Switch and can't be ported or torrented in a PC heavy ecosystem. The saturation point also becomes much higher if they can convince hardcore gamers to double-dip on the mid-gen refresh, which frankly, I'd certainly go for. Then there is also to consider that such a long lifespan would naturally include new children being old enough to get Switch as they grow and normal wear and tear will break some Switches.

To conclude, do I expect the Switch to surpass the PS2? No, I think it will have an eight year cycle, come close and stop around 140-150 million.

But is it possible? Yes, if Nintendo is willing to let it stay on the market long enough, or 2021 ends up being peak year and it blows through my expectations.

I think Switch in the end won't have problems to surpass the 150 million mark, that said PS2 is actually a bit higher than that, closer to 160m.
Soooooooooo not 100% sure it'll become the biggest system of all time but it'll surely give PS2 a challenge.

Do you belive in cliffs ? Is your name Rick ? Do you consider yourself as being quick ?

Gonna be fun revisting this thread and many like it when Switch fails in reaching the PS2 as well. After 3DS and Wii did as well.