You make the fundemental mistake of treating these 3 audiences as if their interests are mutually exclusive. You're treating the worth of the NX as something determined by how well it performs one singular function, versus the fact it can do all 3. Maybe you don't want to spend $250 on a Nintendo home console which will just be used to play mario kart, Smash Bros, Zelda & Splatoon (many people don't, especially when 80m will already have an X1/PS4 by the time NX arrives), maybe you don't want to spend that money on a dedicated handheld (fewer & fewer people are willing to do so), Maybe you like the casual distaction of a tablet but have never cared to put down the money to get a one (many people haven't). Although indvidually you're not drawn to a purchase, perhaps you're comfortable spending that $250 (instead of $750) on one device that does all 3 of these things to a decent degree. It doesn't mean you wont have another console in the house or have another tablet. By their nature, the appeal of multi purpose devices is convience and cost efficency, not excellence in any one particualar field (although Nintendo can offer that where fun is concerned- a factor we can't discount). 

You might casually like the idea of having a tablet, enjoy handheld gaming and love playing Mario Kart with your family. Or you causaually like handheld gaming -mainly just for pokemon & a few other titles, don't really care for tablets but love gaming at home on your TV. 

Regarding your actual points I feel there is a lot of stuff worth questioning.

1.Handheld Market

The 6" Clamshell body of 3DS XL is by far the most popular varient of the the 3DS, so actual evidence suggest bigger is better. Its a given Nintendo will make many games that will be playable with just the touch screen but lets say the attachable controllers make the system 8.5", how do we know thats too big? I personally can count on 1 hand the amount of times this year I've seen somone on the train/bus whip out a 3DS. Everyday however I see people with 8-10" tablets. Secondly I find it curious that a "large" chunk of dedicated gaming crowd who care enough to be irked by detachable controllers are going to turn their back on dedicated handheld gaming entirely (there is no alternative to Nintendo atm) because a system is an inch too big for their trouser pocket. Does a 3DS XL even fit into a childs pocket? 

2. Tablets.

Every Kid I know loves tablets (certainly more then a dedicated gaming device), none of them will care whether the the screen is 720p or 4k as long as it looks pretty (720p looks great on an 8" screen). Very few have their own and I don't know any tablets which are marketed as kid friendly, Nintendo could really score gold here- Imagine the next Pokemon game having an isometric camera mode (essentially classic pokemon) where you can play with touch alone, I think that'd be very appealing for them. Beyond that I think you grossly overstate the function of a tabet for many people. They're trivial, convient devices which are multifunctional. An additional one to the household is always a plus. They don't necessarily act as people primary camera's, communication devices, streaming devices etc

3. Home Console

From Japanese support alone the NX will likely have more games in its first 2 years then the Wii U had in its lifetime.  From what we've heard It'll have, bigger, better games in its first year then the 3DS had by a a large margin. Most importantly NIntendo has much higher potential to make software revenue from all of their software being developed for one device instead of 2 devices with seperate audiences.

I wouldn't pretend to know the NX's fate because I haven't seen it yet, but I think many people are under selling the potential of a hybrid. Of course execution is everything, so its best we just wait to see what they actually have on the table.

It's still not really comparable.

Then we have to take into account that we'll have an ARM CPU on our hands. That thing will most likely be leagues below X1's CPU.

I'm not being unrealistic. Nintendo could easily make a home console that is more powerful than the Xbox One for around $249 or even $199. Polaris 11 is a gpu that puts out around 2.5 TFs, costs $99, and uses little power. Add on the right parts, get the right distribution deals and you got a great little console that puts out solid graphics for around $199-249.

Instead, we're getting a 720p tablet that has a gpu that is 5 times less powerful, has detachable controllers, and comes with a bunch of peripherals to make it work on a TV. And you're expecting $299 for that?

And power may not ensure 3rd party ports, but not having power does. If a 3rd party dev has to pretty much tear their own game apart to make it fly on NX, they won't bother.

It just struck me that if the screen is only 6" theres no reason why there won't a traditional clamshell variant of the system some point down the road, maybe even on day one. The main concern with size is that the rumoured specs can't be housed in a small shell, but a 6" screen can just about fit on the 3DS XL. If the specs can fit in a 7" tablet form factor without overheating, they could fit inside a similar shell to the 3DS XL.

Again you talk only about hardware power, its not just hardware power what makes console. If that is a case why 3DS doesn't have price of $49, it has very weak hardware!?

Of Course, I expecting $249-299 price point, expecting $99-149 (when 3DS is $150) is talking crazy staffs. Again that is a hybrid, home console (that is around 3x stronger than WiiU) and handheld in one, not just Wii U or 3DS, but Wii U and 3DS in same package.

Offcoruse, some 3rd partys will suport NX (you can count almost all Japanese 3rd party), some will not (for instance hardly we will see Bethesda games), but for most developers its most important how much NX will be popular and how much will sale.

You look at NX like just handheld, but its not, its handheld (big) and home console, so you cant expect that will have lower price than 3DS or Vita, lol.

They will through money on console until they think it will paid off. Of Course they will not make new console if they think it will fail. Lol

You realize they're selling the 2DS for $79 right? And that's including a game and memory card. I wouldn't be surprised if they're spending less than $50 to produce a 2DS.

If NX is going to be a 720p tablet, I don't see why they can't make it for $99 or $149.

The problem is outside of a small group of Nintendo loyalists like myself and you, probably not a lot of people would buy such a system. 

It'd be the GameCube all over again, but worse because Sony and MS both have 3+ year head starts, and today Sony/MS also lock people into to their online ecosystems, which is more of a factor than it was in 2001 (ie: even if you prefer say an XBox, you buy the PS4 because your best friend already has it and you want to be able to play COD, NBA2K, Destiny, GTA, etc. online with them). 

Nintendo needed to release the system you've described 2-3 years ago, by 2017 it's way too late. 

If they want to release a "serious" console now, it has to be Scorpio level in spec at minimum, I would say probably even beyond that to really get any mainstream attention. 

2.5 TFLOP isn't going to impress anyone when the PS4 Pro is already 4 TFLOPS and Scorpio is at 6 TFLOP and while the regular PS4 has thousands of games already. The only system it would maybe compare favorably against would maybe be the vanilla XBox One S. 

Well, what do you expect from the successor of the 3DS? To be stronger than PS4NEO, Scorpio, PS5, XBFuture? If the rumors are true, it is stronger than 3DS and Vita - which should be expected.

That is why it has to have a lower price point. $199 and people would love the thing. 2.5 isn't much compared to Scorpio, but it is ahead of PS4 and Xbox One. Then all Nintendo needs to do is ride out the generation with their unified teams, sell on mobile, make some theme park rides and make some more Nintendo merch, and they will have carved out a real solid niche for themselves that doesn't solely rely on video games, but still features it prominently. You can't tell me that a Pokemon with 2.5 TFs of power wouldn't look like the cartoon but in real time. That alone would move units. People will buy a console and Pokemon for $249, but they won't spend $299 for a 720p tablet, no matter who makes it.