How would a joint architecture change the concept of what it means to be a portable or a home console?
It does actually increase the number of devices that can access a particular piece of content, which on the user's side changes the relationship they have with their devices.
You no longer have one device for one slice of content, another device for another slice of content, and some content shared. You just have a big bucket of content and a bunch of choices for how you want to experience it. That's different than how Nintendo has traditionally approached handhelds and consoles.
Where the problem arises is that inevitably the answer to a multi-device architecture where it's just the same games on console and handheld, running at different specs, is that you get something that's virtually identical to what Sony and developers supporting their hardware are already doing:
- Three devices: PS Vita, PS4, PS4 Pro
- Some software developed for all three with virtually the same experience across all
- Pretty much all software available across all through Remote Play
- Some software developed for only PS4 with virtually the same experience on PS4 and PS4 Pro
- Some software developed with special enhancements on PS4 Pro
No matter how scalable your architecture is, inevitably the results that are possible are going to be limited by the performance of each device. And that means some software will inevitably be left behind, or some results are going to be toned down on lower-spec hardware.
So unless Nintendo's literally copying Sony's model with Vita/PS4/Pro (which as others have stated would not be a change from the industry status quo at all given that cross-buy is already a thing across handhelds and consoles), we're going to see something where the relationship between the devices themselves has changed in some way, not just the model for developing software.
And the best answer for that is that you have a device that itself supports a bunch of different form factors and play environments, not a bunch of discrete devices that just share the same pool of software.
I think Nintendo is actually likely to release more than one piece of hardware at one point or another. But, as Satoru Iwata said, they have no idea yet whether one device will be sufficient or whether the market will demand more devices with different configurations.