That's a very well articulated summary of the PC ecosystem. You're right on all points. It's not anything that's news to me, but may be to readers.
Certainly I can imagine that the production chain results in less than optimal default power settings, due the to OEM customization, and especially the smaller OEMs not having the same level of access to the hardware data and support as the larger OEMs.
So, the complexity of hardware and software combinations, combined with inefficient 'end result' oversight, absolutely does leave a lot of room for potential optimization - in either power savings or performance - the latter being where we stepped in, seeing the inadequate handling of core parking and frequency scaling by the OS. Then Intel stepped in, announcing they'd just start doing the frequency scaling on-chip

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Since I spent a few years in embedded linux work, I can tell you that Wireless APs/Routers are very similar. The chipset manufacturer (there are only 2 or 3 big ones) provides the chipset and OEM drivers, template PCB, and a template firmware - these days mostly linux. Anyway, then the retailers like Netgear and Linksys just wrap these chipsets in different chassis and PCBs, combine them with other hardware, develop/adapt their own firmware or extend the stock firmware, and - bam - there's a new router model. Thus, all the brands end up being based on the same chipsets and software drivers. What really differentiates them is the firmware. And *finally* some vendors have gotten good enough with their firmwares that installing OpenWrt isnt the only way to end up with a good router. ASUS, in particular, I've been impressed with. It's funny how little attention some hardware vendors pay to the software, but it's the software that makes the hardware work well, or even not at all.
It's like the classic software and driver issue with PC Printers. Some of them may have great hardware, but without the right software, they end up useless.
Of course, at least in the case of power management, we're just talking about marginal inefficiencies. And that's one word that should be remembered - marginal. How much gain of battery life, in a best case scenario, are we talking about? Nobody may know the answer for sure, but I'm sure it's marginal.
Apple's advantage in having consistent hardware from their closed ecosystem is well documented, though comes with that trade-off of higher prices and less variety. I suppose this 'battle' rages on of PC vs Mac, but I think generally we can say the PC won as far as market share.