There's been a lot of turmoil about Microsoft's decision to cease support for new hardware architectures on its older operating systems. What this means is that with the arrival of Intel's Kaby Lake and AMD's Zen, users will need to upgrade to Windows 10 because Windows 7/8.1 will not be getting the required updates to fully support these new processors.
HotHardware did a little digging and discovered that both Zen and Kaby Lake have new features and technologies that require significant updates to Windows 10 to optimally function. Intel's Kaby Lake introduces a new version of Speed Shift that requires operating system level hooks to function properly. Similarly, AMD's Zen includes significant changes that require support that goes well over basic driver support. For example, Zen introduces new Simultaneous Multi-Threading technology for AMD chips and has fine-grained clock gating with multi-level regions throughout the chip:
For an operating system to optimally support AMD’s Zen-based processors, major updates are likely necessary as well, over and above basic driver support. Zen has fine-grained clock gating with multi-level regions throughout the chip. And Zen will also mark the introduction of newer Simultaneous Multi-Threading technology for AMD chips. To properly leverage the tech in Zen, Microsoft will likely have to make updates to the Windows kernel and system scheduler, which is more involved than a driver update. If you remember, Microsoft had to do something similar to add proper support for Bulldozer-based processors with Windows 7.
So even though nothing will prevent you from installing Windows 7 or 8.x on a system with Kaby Lake or Zen, you will never get the same level of performance, energy efficiency and possibly stability as on a Windows 10 system because Microsoft doesn't want to put in the effort to keep its older operating systems up to date.