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 lack of protected memory was pretty insidious—essentially the kernel relied on application memory for its own internals in MacOS, and reentrancy was a problem for, oh, everything. Copland was to fix that first problem though (but only for new apps).
BeOS was the other option being considered. It was arguably more advanced and powerful than NeXTStep. But Jean Louis Gassée played hardball thinking he had Apple on the ropes, and they went with NeXT instead.
