From where did this implicit expectation come from?

Windows 7 mainstream support ended in Jan 2015 and that was known since forever.

Extended support implies:

>Microsoft no longer supplies non-security hotfixes unless you have an extended support agreement

>All warranty claims end

>Microsoft no long accepts requests for new features and design changes

Even a very old system image of windows 7 will run on the very latest skylake. It doesn't need the very latest updates. I doubt they will create a kill switch in a windows update that refuses to boot if the system detects a too modern CPU.