They haven't always! DS and 3DS (3DS especially) had very soft launches.
whats's the numbers for 3ds, i read some where they are close to switch, not that far off. regardless, soft launch, or strong one doesn't mean much, the product was in demand at a lower price, unlike n64, GC, and wiiu.
Defining the Switch as a handheld for the purpose of taking for granted that it would sell well on the basis that past handhelds have sold well is kinda ridiculous. The technology, price point, and games all resemble a Wii U successor much more than a 3DS one, and a good amount of people are going to be leaving it docked a lot of the time. Even putting aside that the 3DS looked like it could have been a dismal failure at one point, nothing about the handheldness of the Switch guaranteed success, just like nothing about the consoleness of it did.
I disagree a wiiu successor would have been at least as powerful as a ps4. this is what a handheld should be in 2017, 3ds launched at 249$, not seeing a huge difference in price, especially for what you're getting. as for 3ds looking like it was gonna be a failure, it wasn't, it just wasn't priced right, just like ps3, people wanted the product.