For reference, the 3DS launched with 240p 5 years ago... when you claim 720p was already becoming the standard. Nintendo has consistently been far behind the curve with their handheld screens with only meager jumps in resolution each gen, and the idea that they're suddenly going to jump to HD for a dedicated handheld just for the sake of having it doesn't make sense to me. They're not going to drive the costs up and make a more expensive device just for the luxury of having an HD screen- this company has made it clear that they don't do things like that. If NX is going to include a console, I'd expect the handheld to be a little more modest. It could still run console games with a 540p or 480p screen, for instance.
And if they're going to have a dedicated console... why bother connecting the handheld to the TV at all? Why put so much emphasis on using the handheld as a console if they're going to specifically design a console to perform that same function and play the same games? Wouldn't it undercut that hypothetical later-releasing console's appeal if the handheld could already give people that same function? The only real need to buy the console would be more power, but again, Nintendo is absolutely not the kind of company that caters to that kind of thing.
I'm not saying it's impossible, I'm just saying it doesn't seem plausible that Nintendo would shoulder the extra expense and release a high-powered handheld that can function as a home console if they were also going to release a console dedicated to doing that same thing. If they're going to have two systems, why not keep them as focused and cost-efficient as possible? Why so much unneeded overlap?
3DS was already way too far into development to change anything by the time larger-screened phones with high-res displays started taking off in 2011. This is the first portable Nintendo has designed from the ground up since the smartphone boom, and it's not surprising at all for it to have a pixel density that's at least somewhat passable by today's standards.
We can't comment on the how much emphasis there will be on TV play in general until the system is officially revealed, but based on these rumors, there is very little emphasis on playing this device on the TV from a hardware perspective. We've heard nothing about a high-powered mode when the system is docked or a more traditional (optional) controller that is designed for home use. Nearly all of the NX's gimmicks are based around being able to play it anywhere. The biggest innovation here (if that's all there is to it) is that the detachable controls could function as two mini-Wiimotes, two simplified sideways controllers, one Wiimote/Nunchuck combo or a set of traditional controls. In contrast, the TV functionality just seems to be a charging cradle with a couple USB ports and HDMI out. According to some reports, you may not even need the dock at all, which is similar to what the PSP was doing years ago. However, all of the above, coupled with the multi-touch screen effectively allows the NX to have the broadest range of gameplay possibilities of any platform, regardless of it being used at home or on the go.
That's why this device makes sense even if there are other form factors coming later on - it's the best way to give anyone a taste of everything Nintendo has to offer for a supposedly very affordable price. You've got;
- Single-player touchscreen gaming (screen only)
- Single-player motion gaming (one paddle or both)
- Single-player traditional gaming (both paddles attached to the display)
- Two-player motion gaming (paddles held like a Wiimote)
- Two-player traditional-ish gaming (paddles held sideways)
- Some combination of the above
And all of these would be possible anywhere. This could grab the attention of people who aren't being served by Nintendo's current offerings. If the system is a success, they could branch out into more specialized types of hardware, all based on the same architecture and software development pipeline. And with the first (and probably weakest) device in the lineup supposedly being relatively close to the Xbox One in real world performance and supporting some of the most popular engines and dev tools, both first and third parties wouldn't have to compromise for a very long time.