I think the primary difference is marketing; PSP and Vita weren't marketed for their ability to offer both on-the-go and at-home-on-your-TV play. It was treated as a kind of throwaway feature, while with Switch, this capability is being marketed as the system's core ideal. It's more a matter of advertising than functionality.

It's not so much about Nintendo being able to afford to make another console. They could obviously.

The question is if Switch fails, then basically the consumer has told Nintendo they don't want home consoles from them (Wii U) and they don't even want portables from them either.

The market will basically consolidate entirely under the Sony or MS camps. Also Nintendo's internal business structure would step in. This isn't a hobbyist club where designers get to do whatever they want. Most likely if Switch fails, a lot Nintendo's suits will start to push for a hardware-less approach on mobile devices where maybe you can just buy "Joy-Con" like snap ons for your smartphone or smart tablet and play Nintendo games that way (no need for a seperate piece of hardware). 

Offcourse you are wrong, primary difrence is that PSP/Vita are not built on first place to be used like real home consoles also, they are just handhelds that have just ability to be conected to TV (PSP at least have, I am not sure that Vita has ability to play Vita games on TV at all), while Switch is built to be used like real home console and real handheld in same time, it's built that operate like that, thats why you have dock and you have separate controller in package and thats why you have full home console expariance with Switch out of box (so of course that is about functionality), and thats why Nintendo is marketing like that, you cant say that for PSP and Vita.

By no meaningful criteria is Switch any more of a "real home console" than a PSP connected to your television. They both offer the ability to game on the go or on a TV. Only real difference is Nintendo is centering their branding around this feature while Sony did not.

Again you are wrong, PSP let you play games on just half of TV screen, also you using PSP itself like actual controller, and you can't play games in local multiplayer with PSP on TV, at end of day PSP is just a handheld that can be connected to TV with cable and nothing more. Again wrong, difference is that actually Switch is built in order so it can be used and that act like real home console also with all home console features out of box, while PSP is not built like that at all.

Resolution doesn't define what a platform is, a separate controller is not needed as the PSP serves as one, and local multiplayer is diminishing in relevance. None of these things are gamechangers, the core idea is that it can be played either portably or on a TV, and PSP does that. Switch is simply expands on this concept and centres its marketing around it.

Who said that resolution define what platform is!? But on every home console you can play on TV on full screen not on half of screen, on evre home console you have separate controller not actually using device itself for controlling games, on every console you have support for more controllers and you can play local multiplayer, with Switch you even have dock (it gives power and picture for playing in home console mode) which is used when you using Switch like home console, PSP can't do anything of those things.

You can keep denying that, but with Switch you get also a full home console experience out of box, because Switch it's actualy made with that purpose, to act like real home console and like real handheld, hybrid, with PSP you dont get that.

DOOOOOOOOOOOOOOOOOM!

Seriously though, this is the least original possible topic you can make. These have been around since N64.

Not only it's true but as a handheld Switch has 2 issues: it's quite too big (tablet size) and I assume it's going to be too expensive.

It really isnt that big, most people cant fit handhelds in their pockets very comfortably anyway.

Most rumors/leaks seem to be pointing at a $250 price tag which seems appropriate for what it is.