Because Microsoft didn't write the drivers for 100% of hardware... and tbh, if a company wants they can pay Microsoft to write those initial drivers, and provided the hardware for testing, iirc, they tend to maintain those drivers between versions. There are plenty of drivers that are written by MS, it's pretty common with Printers iirc, and works pretty damned well.
Because foo pro-audio, or bar cnc-board didn't want to update their drivers, or for that matter, more often than not, had a weird installer that simply didn't follow proper practices, doesn't work in the new version of Windows isn't MS' fault.
Nevertheless, because the chance of something like that happening is clearly higher than 0.0000000001%, Microsoft should simply never do such a drastic upgrade automatically. (So, even within Windows 10, they should not automate all upgrades, because occasionally they will have to include bigger changes.)
Also, users will not understand this, and will, rightly, blame Microsoft.
Any trivial update has a higher than 0.0000000001% chance of failure... if you need that level of specificity, you should disable updates, and network connectivity altogether.
Because foo pro-audio, or bar cnc-board didn't want to update their drivers, or for that matter, more often than not, had a weird installer that simply didn't follow proper practices, doesn't work in the new version of Windows isn't MS' fault.