[kergonath]

> Apple had no money or time to invest in rewriting NeXTStep for a completely new kernel they had no experience in.

I broadly agree, but it is more nuanced than that. They actually had experience with Linux. Shortly before acquiring NeXT, they did the opposite of what you mentioned and ported Linux to the Mach microkernel for their MkLinux OS. It was cancelled at some point, but had things turned a bit differently, it could have ended up more important than it actually did.