Interesting thread! I thought I'd put in my two cents.
I agree that the PC's open architecture was the decisive point. There wasn't any crucial leveraging in place like Jack T. had with the C-64, where he was basically providing his own components at cost.
As I see it, the best bet for Commodore was the Video Toaster. They might have put all their eggs in that basket and won over hobby and independent video editors and hung on for a long time. I think Mac succeeded like that with graphic designers.
Still, the big problem was that AGA, 16-bit sound, and CD-ROM just took too long. If they could have gotten that technology out three or four years earlier, it could have proven decisive. Imagine what might have happened if the 1200 had arrived in 1988, for instance.
One of my favorite scenarios is what might have happened if Commodore had licensed out its Amiga OS and let all component/computer manufacturers compete to provide cheaper and better systems. There was still a gap there between DOS and Windows 95 that could have been filled quite nicely. Of course, not sure how the Motorolas could have competed with the Intels at that point, especially after the Pentium in 93.