Chip manufacturers bent over backwards to make sure their chips ran Windows perfectly. This is obviously changing and they are not investing as much as they used to in compatibility with existing software.
Err, no. This is 100% Microsoft trying to kill all Windows prior to Win10 by making it virtually impossible to continue to use the older Windows versions (i.e. by blocking updates and removing support for new hardware).
There's nothing technically different enough about these new CPUs that anything significant needs to be done by Microsoft - in fact, it took more code to deny the ability to install on these new CPUs than it would take to allow it.
The 80186 was a market failure because, mostly, PCs built with it were not completely compatible with software already existing. This was because the components the 186 integrated were not copies of their IBM PC analogs. IIRC, at that time, the "gold standard" of PC-compatibility was Flight Simulator: if anything was even slightly different, it wouldn't run. Many PC almost-compatibles had special versions of it.
Intel learned their lesson and all subsequent processors were crafted to be exquisitely compatible with all processors that came before them and to make it easy to build computers that ran all previous PC software. AMD64 (the architecture) is limited by the need to run x86 code seamlessly. Everything is there, from string copy instructions , x87, MMX, SSEx, either explicitly or as parts of newer standards that superseded them. Until recently, every PC chipset had, deep in it, a fully functional ISA bus.