A/UX isn’t Mac OS 8.
Your reasoning also isn’t sufficient. The classic Microsoft Kernel was able to support a much wider variety of hardware because it was modular (internally, not architecturally). The classic Macintosh Kernel had a far smaller ecosystem to support and couldn’t even add support for hardware that existed on many of their own devices that would make the kernel on-par with the 9x kernel and be transparent (except for improved usability) for users.
So to recap, OP claimed that OS 8 (not A/UX) was superior to 9x. And that’s simply false. Many consider the 98 kernel garbage, even for its time; and yet it’s objectively better to OS 8’s.
If we’re simply arguing Apple hardware OSes versus Intel options (as you seem to be conflating this to), then the latter still wins with Xenix, any number of Unices, BSD, Linux, etc; all more stable and better supported than A/UX as well as better UI-centered OSes (NT, OS/2, BeOS, XFree86, etc).