alt Hacker NewsAssuming the Mac folks had no interest in converging the platform in favour of the Lisa is somewhat unfair. While it sounds like some code was shared between the two platforms, the Lisa's operating system was quite different. It would have been difficult to make Lisa software operate under the Macintosh System Software. To my knowledge, there was virtually no software for the Lisa anyhow. Breaking software compatibility on the Macintosh to get the benefits of Lisa would have been a terrible business decision.
Aside from that, the MMU in the Lisa would have been a custom solution which Apple would have to support. When Motorola introduced an MMU, it was for 68020 generation machines. Apple should have been able to introduce memory protection at that point, but didn't. One of the reasons was that Apple struggled to make that next generation operating system while retaining compatibility with existing software (albeit, memory protection may have been only one of many problems). This was by no means a problem exclusive to Apple. Other platforms ran into similar issues.
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That delay in shipping a memory-protected Mac was probably originally at least as much the result of upper-management politics as anything else. After Jobs left Apple Gassée cancelled Jobs’ pet project, the Big Mac which was intended to run Mac applications on a Unix base. Big Mac project leader Rich Page (and IIRC some other project members) rang Steve Jobs begging him to do something, and the rest is history.
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.