I imagine it would have looked like a very, very well-designed iPhone app: soft and welcoming, yet with delightful new features that became apparent with use, and serious functionality that would stay out of the way until it was needed.
I bet MacPaint would have looked a lot like Paper or Procreate.
It's a very interesting question, and honestly, I don't know how good it would have looked. I know what a lot of websites (including Apple's) looked like when designers had access to full color, bevels, drop shadows, etc. They looked terrible. I think those technological limitations—no color, almost no RAM, no hard disk—are one reason the original Mac GUI looked as good as it did. There just wasn't room for anything else.
That said, the primary reason why the first Mac OS was so tasteful was because Susan Kare is an excellent designer, and would probably not have made the mistakes I described above. (The Lisa had all those limitations too, and an extremely similar feature set, but if you compare the Lisa GUI to the Mac's, it looks a lot rougher and more inelegant. Kare was not involved with the Lisa, and it shows.)
So who knows. I'd love to see someone reimagine the original Mac OS using a more contemporary design language; would be an interesting experiment.
(Also, if you meant Bruce Horne, he was a software engineer; he wasn't involved with the visual design as far as I know.)
I'm not sure that the Spatial Finder needed to die; the standard excuses (we have too many files! our displays are too large!) feel knee-jerk and not well thought out at all.
And the Bruce I was referring to was Bruce Tognazzini.