How could Apple could have gotten a real OS more quickly? The Mac OS of the time had no preemptive multitasking or protected memory, so software running on it was doomed to be made unstable by all the other software. NextStep, maybe it was what you say, but they needed to make a change.

The big advantage of NeXT was the development tooling, it was streets ahead of almost anything else. As a unix it was nothing to write home about, but as a desktop OS it didn't need to be. Just being a Unix at all with decent networking was a huge advantage compared to all the misconceived desktop OS projects at MS, IBM and Apple at the time.