As a NeXT developer who started the NeXT User Group at my college, I agree they were lovely machines, way ahead of their time.
But I want to be quite clear that as a business, NeXT failed. They started out wanting to be an integrated hardware/software company, like Apple. But Jobsian arrogance entirely undercut that; it was a machine developed (and priced) for people like Jobs, so it never found market acceptance outside of a few market niches like finance, where people had money to burn.
Eventually they gave up on hardware and shifted to being an operating system on other people's hardware: Sun, HP, and eventually, a narrow set of Intel-based PCs. This kept them going a little longer, and made things easier for their existing customers doing in-house development, but they were still failing more slowly.
They then pivoted toward being developer tools for Windows machines, except they could only run on the more-expensive Windows NT. This too was headed to eventual failure.
Apple did screw up writing a modern operating system enough that they decided to look at buying one, and I know Be and NeXT were candidates. But I don't think the deciding factor was technical. What Apple's board wanted was Jobs, so I think of the NeXT acquisition as more of an acquihire with some bonus technology.
Yes, Jobs and his investors eventually turned a modest profit on the sale. But Apple quickly shut down all of the NeXT stuff, leaving NeXT developers like me high and dry, which is a strong sign it was not a viable business. So I look at the sale as Jobs's price for rejoining Apple, his way of not admitting failure.