One could imagine a sophisticated attack involving multiple patches to different subsystems at different times, that eventually converge into a Voltron-like monster once they’re all inside the kernel.
Potentially. However that also leaves a larger attack-surface, in the sense that more patches = even more scrutiny. Conceivably such a scheme would require all parts to be included (I mean, why make it larger than necessary?), so it would be extremely fragile to any single maintainer seeing the potential for abuse and correcting it.
There are far more devices running Linux in the world than Windows & macOS combined. That would seem to be a much more useful comparison than how many people own those devices.
Parent post was updated to say "not counting servers and embedded", but dismissing smartphones -- literally the most popular type of personal computing device in the world -- this way seems to completely invalidate the point.
The parent thread was about possible backdoors; more devices running a piece of software = more surface area for any possible backdoor in that software.
