We all would like backwards comparability, but it comes with a cost. Keeping the PowerPC line requires too many compromises on Nintendo's part that have a negative effect on the performance of games for the current generation. The PowerPC chips in the Wii U couldn't keep up with the CPU's of consoles that were seven years old at the Wii U's launch.
We're not celebrating the loss of backwards compatibility- we're celebrating the removal of the handicaps that seeking BC for this particular line of chips brings with it.
The console having BC doesn't force it to use PowerPC for the system CPU. By far the most likely scenario would be an ARM-based SoC as predicted, and then a separate Espresso die for BC (and maybe to offload some OS functions onto).
To be honest I see the controller screen being by far the bigger encumbrance from BC. Incorporating a screen and the other components you need to accommodate it (e.g. larger battery, streaming hardware, etc.) would add substantially to the cost of an NX home console, far more than a little Espresso die.
I also think, coming back to my point about accommodating NX's successors, that the philosophy for the design of the NX hardware (both handheld and home console) should be to keep them as simple as possible to allow them to be more easily iterated upon. If the NX has a screen in the controller, then every future NX has to have a screen in the controller. If the NX has split memory pools then they're constraining their future selves when developing the next NX's memory system. If the handheld NX has two screens, then every future handheld NX has to have two screens, etc., etc.
I don't necessarily mean that I think that's what Nintendo will do (as they're not ones to be simple and predictable), but if I were designing the NX home console and handheld, a lot of my focus, from the guts of the console to the control method, would be on making sure I'm not setting up roadblocks for myself five years down the line when I have to build the successor to this thing.
I could see the handheld coming first due to the age of the system, in fact I wouldn't be surprised if the home console is being rushed and the proposal for having the handheld and home console be similar/streamlined being spurred from this increased pressure to get a home console out asap due to the Wii U flopping. They must be doing crazy hours at Nintendo to get this shit ready for next year never mind this year.
Iwata talked about integrating software development for handhelds and home consoles as early as January 2013 (it's the very first thing in the OP). The contract with AMD for the design of the console's APU looks to have been signed around November 2014, with design work starting November/December 2014, which would give them ample time to put together a home console for a late 2016 launch.
Yeah, good point on the memory controller. I should've thought of that. I was just speculating since I saw that 16 Gigabit DDR4 chips seem to be available and a last minute swap from 4 GB total external RAM to 8 GB might be a possibility, as you said. Not the bus, though, so it wouldn't make sense if they already decided on a 128-bit bus. They would have needed 8 chips for that and doubling capacity from 8 GB (8x8 gigabit chips) to 16 GB (8x16 gigabit chips) seems excessive (I'm also unsure of what speed those 16 Gigabit chips run at...probably not 3200 Mbps like we'd hope for).
Oh, and I don't think a small form factor would be a sudden decision. Rather, like Wii U, they'd have a box size decided on and then gradually see how much of a clock increase they can get away with before it sets fire.
If NX came in a Xbone sized case with vents up the wazoo, I can't imagine this being much of an issue.
Well, yes, they could be switching from 4GB to 8GB, or even from 6GB to 12GB for all we know. Trying to guess sizes or bus widths is incredibly difficult, though, without anything to go on. Particularly as we can't even say for sure whether they're using split pools or not (Wii U was easier as it had BC with Wii, which pretty much necessitated split pools). For all we know they've said to hell with split pools, let's go with a single pool of 6GB GDDR5 on a 193 bit bus to simplify development. Hell, for all we know they've thrown the entire console budget into HBM just because they love having memory so close to the CPU and GPU.
Also, on DDR4 speeds, I'm pretty sure only 4Gb chips will clock to 3200 MT/s. The 16Gb chips seem to be limited to 2400 MT/s.