Apple doesn't seem to have leveraged or combined work on (Lisa, Lisa Smalltalk, Lisa Xenix, Mac OS, A/UX, ...) as successfully as they might have. As you note, protected memory was deferred to multiple failed Mac OS successor projects (Pink/Taligent, Copland/NuKernel, etc.) Ultimately Apple gave up, acquired Steve Jobs and NeXT, and eventually successfully migrated the Mac platform to an OS with memory protection.

Since then however Apple's OS and hardware strategy has been much more coherent, with macOS, iOS, iPadOS, tvOS, watchOS etc. sharing code, and sharing SoC technology as well. Ironically this is similar to Microsoft's "Windows [NT] everywhere" strategy.