There was never a right time for Apple to make such a switch. NeXTSTEP predates Linux, and when it was adapted into Mac OS X, Apple couldn't afford a wholesale kernel replacement project on top of everything else, and Linux in the late 1990s was far from being an obviously superior choice. Once they were a few versions in to OS X and solidly established as the most successful UNIX-like OS for consumer PCs, switching to a Linux base would have been an expensive risk with very little short-term upside.
Maybe if Apple had been able to keep classic MacOS going five years longer, or Linux had matured five years earlier, the OS X transition could have been very different. But throwing out XNU in favor of a pre-2.6 Linux kernel wouldn't have made much sense.
Maybe if Apple had been able to keep classic MacOS going five years longer, or Linux had matured five years earlier, the OS X transition could have been very different. But throwing out XNU in favor of a pre-2.6 Linux kernel wouldn't have made much sense.