That has had me rethinking your what-if. I don't think the unix is a big deal - the Be and Haiku terminals feel unixy, and BeOS offered a big subset of the unix system calls even into v3 (time of Apple purchase).
By v4.0 (two years after), BeOS had become a remarkable system for single-user, close-to-the-metal computing. The user experience was very fast and stable. The system gave a skilled developer easy access to video and audio.
I've been trying out haiku again this morning. They've done good work. It now boots on my thinkpad, but, like OSX or Windows, feels sluggish in a way that Be never did. It needs some serious performance tuning. They need to get their out-of-the-box development experience sorted out too. OSX has done an excellent job of that.
I really think it was the NeXT engineer's tech demos showing that they could run Openstep on different processors and also that they could complete the 'Blue Box' in time.
Not sure how that would have gone with all the geeks that went for Mac OS X without an Apple background, just because it is UNIX.