of Windows and Skylake support: Windows is the only OS that reliably offers support for old (no-longer-shipping) versions of the OS on new hardware. I don't think it's unreasonable or even very surprising that they're looking to restrict this support to business customers using business hardware. That said, a fair amount of business hardware is supported and in general, good business hardware costs about what good consumer hardware does, so if you know in advance you want to get a few more years out of Windows 7 or 8.1U1, then it's not hard to do.
I don't think most home users buying pre-built systems in physical retail stores really care, though.
Ok, so storage hasn't kept pace with Moore's law, but that doesn't explain why OS I/O requirements have exploded. I can easily explain why the number of transistors in CPUs and GPUs doubles every 2 years, but no one can reaally explain all the reasons why commercial OSes always immediately use that new silicon to make heat instead of do work.
Disk caching, swap activity, launching applications, using ever-larger data files that must be kept in ever larger quantities, along with maintenance tasks such as TRIM and defragmentation.
Outside of Mac OS X (which, from about 10.6 or 10.7 to 10.10, was SSD-mandatory because it would enter swap about 6 nanoseconds after you booted, even if you had sixteen or sixty four gigabytes of RAM), most operating systems don't spend a whole bunch of time hitting the disk without a good reason. In fact, Windows 10 is honestly really good about disk activity. You can pretty much be assured that if it's hitting the disk, it's because you asked it to or because it's doing a maintenance task designed to make your life better.
For that reason, Windows 7/8/10 are very usable on old spinning disks. You're still going to have problems if your disk is dying, but generally when reviewers write things such as "eMMC is so terrible" when talking about low end computers, what they mean is that their personal systems have much faster storage and they notice the difference.
What new workload does 10.11 do that your 10.8 didn't do? My googling reveals such improves as Mail.app has tabs now and you can do Windows 7 window tiling...
Almost nothing. Well, except for the whole part where 10.11 makes my Macs feel like what they are: my fastest and newest computers. Full screen and side-by-side full screen operation are potentially important for people who came to the Mac by way of iPad, or people using the smallest of Macs who want to dedicate all available pixels to a particular app.
I agree completely with what I think the core of your suggestion here is -- that Apple should slow down and really improve the core of Mac OS X for a little bit.
That said, just because you don't personally value the new features in a version of Mac OS X doesn't mean that you shouldn't upgrade. In general, Apple out of hand refuses to fix even the most egregious of security vulnerabilities in the directly previous version. As of several months ago, there are known vulnerabilities in 10.10 that Apple has yet to fix, which have been fixed in 10.11.
As IPalindromeI says, it may be best to avoid Mac OS X if you prefer longer support cycles without changing your OS. On the other hand, the good thing about Mac OS X upgrade cycles (and how easy it is to image Mac disks) is that you can usually install the new upgrade and then keep using your system as though nothing had changed, without engaging or using any of the new features, which can often be completely disabled anyway.