I have argued, how I can't see it reaching PS2 and DS, but I realized a main point of your is adding together PS4 and 3DS sales. I feel like I should address this.
So, it sits not too well with me, just lumping together sales of two consoles. It assumes there is one homogenic blob of home console players and one equally homogenic blob of handheld players. The overlap was already adressed in other posts, but I don't even think the overlap is the most important point here.
Every slightly successful product does not only address one target audience, it adresses multiple audiences. This is true for successful consoles. It is a reason BTW, why I can't see PS4 reaching PS2 numbers, as they at least miss out on the casual crowd with Dance Dance Revolution back in the day, and cannot recoup that on other areas. The same is true for Switch.
See, if you add the numbers together, you just assume every home console user and every handheld user is drawn in, just because Switch can be used as a home console or as a handheld. But each of the two groups has differing audiences. And as I said before Switch is designed to address multiple audiences, but it still does not address everyone.
Let's start with some audiences of the PS4. This includes guys going for high end graphic and power. The Switch never can match that. It can't match the power of the PS4, even less of the PS5. Even if we compare with WiiU, the Switch don't make a step up in power you would expect for a pure home console. As some don't care about mobility, this does matter for these target groups. But given, the power junkies are a small group. But bigger is the group choosing he best platform for the multiplatform game they like (instead of going for the version of the game on the platform they own for other reasons). Switch cannot outdo FIFA, COD or AC on PS4, even less on PS5. Look at the differing specs of Dark Souls Remastered. Sure, the Switch-version has mobile, but as I said that is a different target audience. The audience looking for the best graphical advanced version of the game will not get Switch.
There are more target groups purchasing the PS4. Each not big, but they add up. This includes the achievement-fanatics and the online voice chat crowd, something the Switch isn't supporting (so far?). And we have networking effects. In case of consoles: you purchase a console your friends have. Especially if you want to play together with them.
But a biggie are the Playstation enthusiastics. People underestimate brand power. You included the Nintendo fanatics with 3DS, but also the Playstation fanatics with PS4. This will not work that way. Nintendo can work on increasing their brand power, but they just cna't capture the crowd following another brand. So this is also missing.
On the 3Ds-side of things you lose also some audiences. I can see for myself: I have my 3DS with me on a daily base. The Switch only on bigger and longer travels. It is bigger, more clunky. While the 3DS fits in many pockets, the Switch does not. The clamshell-design is a big pro for the 3DS here. So this reduced mobility will be important for some in the 3DS crowd, so they'll skip Switch.
Also not to forget price. Handhelds usually are pretty affordable, which helps in purchasing multiple devices for one household. The Switch price is more prohibitive. And yes, price-cuts are possible, but so they were for 3DS, which mean it has lowered it's price over time and probably will do so at least one other time. If your price starts higher, it'll end up higher probably too. This'll exclude some more.
