Re: when Secure Boot can't be turned off
If you're talking motherboards, I think you're wrong. Makers of motherboards to be sold on their own and have a system built around them are much more responsive to what their users want-- they know that people who buy their stuff are very aware of things like configurability and the ability to install older OSs on newer hardware. Several motherboard makers right now, for example, have included workarounds in software or firmware to get around the issue of Skylake's lack of EHCI support and the Windows 7 installer's insistence on EHCI support for USB devices pre-boot.
This gives people like me the ability to work around all of Microsoft's silliness. Secure boot is not going to be locked on in these boards as it could be with a brand new prebuilt machine. My board uses UEFI, but it has the option of using legacy BIOS mode for the boot process for maximum flexibility. I can't imagine (as an individual) buying a branded desktop PC as a whole unit-- I've never done it in the 26 years I have been using and working on PCs.
Laptops, though, are another story (and desktop PCs for enterprise would be much the same). Building one from components is not an option. In that case, it will require extra diligence and research to make sure the one you select has the options in UEFI to accommodate future upgrades, downgrades, or whatever else you want. If people want it, PC OEMs will make sure we have the option to buy it.