Most important of all, and in addition to OPs arguments, is the fact that neither Windows nor MacOS are proper realtime operating-systems and thus not fit for purpose as a DAW-platform. It is important to have complete control over what userspace-applications do, but guaranteed response-times for time-critical operations (drivers, sync etc) is essential for anything that has to do with audio or video editing.
On top of this it is a sad fact that many DAW-computers are not properly maintained. Both Windows and MacOS have their software-update facilities, but neither offer an open solution for third parties to use without exorbitant fees. 3rd-party developers end up building their own infrastructure for updates and users end up with as many mechanisms for software upgrades as there are 3rd-party software-packages on the computer. Updating that should be one single user-task becomes a process that takes half a day or more. Contrast that to for example the software packaging and distribution in Debian (+ubuntu and others) where any vendor can operate their own package-repository and have their products updated alongside everything else on the users computer.
