In the places where there's something entirely different going on, such as networking in earlier BeOS, it's trash and you'd obviously want to throw it away and do what Unix did - Be were doing so. So a hypothetical turn of the century fruit-flavour BeOS is probably much closer to "Unix with a different UI" than you're thinking. NeXT changed nothing there.
The amount of work done by Haiku to provide a proper POSIX userspace support proves the point.