People say this, and I'm 100% sure it's conjecture. Like, what are you basing this on? Who are these "people" who would find this so difficult?
My point is that the 3DS, while slightly smaller than the Switch, managed to exist just fine as a "portable" system, and people didn't seem to have aneurysms trying to figure out how to carry it around.
I just don't know what's so difficult, and to everyone speaking on behalf of what "the people" want, I'd like to see how your personal market research stacks up to Nintendo's.
I mean the ENTIRE appeal of smartphones (and tablets) is that it's an all in one device. You can handle all your communication needs (phone, texts, e-mail, facebook messenger, skype and every other communications app), be your music player, give you games to kill time when stuck in line or on plane, be your camera, be your calendar/PDA etc.
As such it's greatly shrank dedicated device markets like stand alone MP3 players, PDAs, lower end digital cameras (pocket cameras). They're doing the same with dedicated gaming portables, with just more core gamers still buying them (like more serious photography people buying nice digital cameras), and a lot of the more casual players (people that loved Tetris or Brain Age or Nintendogs) finding the games good enough. And Nintendo could hurt themselves there with some former fans finding their mobile apps "good enough" to scratch their itch. Why would they pay $300 for a Switch and lug it around if there phone is a good enough time waster?
I agree the size over 3DS isn't a barrier for the serious portable gamer. But at the same time, I work at a University with over 30,000 full time students on my campus and I haven't seen anyone playing a 3DS or Vita in a couple of years, while people were (and still are but fewer) catching Pokemon all over the place. So I think a lot of portable use is just at home/on out of town trips as people just don't like lugging gadgets around. Unless you've got a long public transit commute, most people don't have the downtime out and about to make lugging a portable around worth it. Much less a bigger one.
As for market research, Nintendo's is clearly fucking terrible at that outside of the magic they pulled off with the Wii and DS. Makes me wonder if they just got lucky that time or if those people left the company.
Ignoring that post NES they've:
-Had a failed partnership with Sony that give birth to their most dominant competitor
-Stuck with carts in the CD era
-Went with proprietary discs for the GameCube, when DVDs were exploding and helped sell PS2s like crazy
-Named the Wii U the Wii U, thought people would be attracted to that "tablet" thought there was a burning desire for offtv play in the West (especially US) where people have big houses and multiple TVs etc.
How do you know so much? You can see the future? I'm just wondering because you don't know how it will play out.
Of course I'm just speculating like everyone else.

I find this fun, and will be fun to look back down the road and see how right or wrong we all were.
When the iPhone launched there was one SKU. Now there are like 6-10 iOS SKUs in Apple's lineup at any given moment.
I don't have any reason to believe that launching with one SKU means they've deviated from the original plan.
True. Maybe this was the original plan. Just seems risky to me as it's so flawed for everyone but hardcore people that want to game on TV and portables. For the console only crowd it's overpriced, underpowered, lacking storage etc. For the portable only it's very big compared to past portables, has middling battery life and is also overpriced at $300 given the Vita and 3DS failed to sell at $250 (and portable only folk don't care about console mode).
But only time will tell. Maybe it will do gangbusters and the concept has mass appeal among millineals that I"m admittedly (and fucking willingly) out of touch with. And/or there's enough suckers like me that love Nintendo games enough to bend over again and overpay for hardware we dislike to play their top notch games (though that clearly didn't work for Wii U).