Yeah go ahead and release it for tablets, see how that goes. But let me clue you in: it would be a waste of time and money. A huge waste.
And clearly the Sony brand doesn't carry that much weight in Japan or the Vita wouldn't have gotten annihilated there. And as for the rest of the world, it would go worse, far worse.
Waste of what? It would be virtually no resource cost for Sony, it's just a controller, they make tens of millions of controllers every year. It would be a minimal investment on their part.
Tablets already sell (far in excess of the 3DS or even Nintendo DS), so there obviously are users out there, I think some people would prefer to pay only say $70 to get a Switch like experience instead of $350 for one with a game.
I would hope they don't do it, but if I was running Sony I'd probably do it just to make life a little more difficult for Nintendo. The point doesn't even have to be for it to be a huge success, all it has to do is muddy the waters so the Switch is not so unique, once Move and Kinect were released, Wii sales declined rapidly, maybe that was coincidence or maybe it had something to do with the fact that the Wiimote was no longer really anything unique. You could get that same expereince in different ways.
Well now you make it sound like a "just launch it and let it do its thing" situation. If they did that no it wouldn't cost much but it would have even less of an impact. They would need to do a lot of marketing to get any attention away from the Switch. No marketing=no market impact, simple as that.
And his wouldn't provide a Switch like experience at all, it's just a set of controllers that you attach to your phone that will have to deal with all the niggles the numerous other such controllers do. And this isn't the late 2000s anymore, the primary dividing line between mobile and handhelds have long, long ceased to be lack of physical inputs. That's still a big component, but it's now more about the ecosystem and experiences found on mobile. And a pair of controllers won't change that at all. Nor will it change how horrendously unoptimized the mobile OSs are for gaming (or anything in general) or any of the other issues. And it certainly doesn't improve the process of hooking your phone or tablet up to your TV.
My point is, this option you seem to think will impact anything has been around for YEARS. If people gave a crap then someone would have found success with a product of this nature. This wouldn't muddy anything because it wouldn't emulate what the Switch provides in any way.
Oh and the fact more people buy a tablet - a multipurpose computer that happens to give access to a freemium infested, casualized market but for which gaming is and remains a secondary or even tertiary function - than buy handheld game systems - specialized gaming only devices that specifically caters to those interested in premium dedicated gaming experiences - is not shocking. And those who buy a tablet OVER a handheld and thus have no interest in a dedicated premium gaming experience - you know, the experiences provided by ALL dedicated gaming devices by virtue of their design - are not part of the Switch's target audience. And would incidentally not care to "hardcore up" their gaming with attachable controllers. And those who are interested in dedicated premium gaming experiences as seen on the Switch, PS4, etc will not suddenly be swayed to leave that behind because of some attached buttons on their tablet. That's not a "Switch like experience", it would be about as good a substitute as a kids tricycle for a Harley .
Oh and the Wii sales decline probably had more to do with Nintendo's releases slowing, their marketing scaling back to a huge degree, third party support falling off a cliff, the real world performance gap becoming more and more glaring between the Wii and PS360, the continued rise to priminence of western developers Nintendo didn't have on their side (Bethesda Game Studios being a prime example), motion controls falling out of favor due to shovelware and the fact of their contextual usefulness, there's MANY reasons the Wii began to decline, the Move and Kinect seem pretty low on the list.