Planar.
Bitplaned graphics was definitely an impediment to efficient rasterization. But when you look at what demo sceners have done with 3D on the Amiga, it's hard to argue that was its main downfall.
What killed the Amiga was stagnation in both technology and marketing. It also never had much of a foothold as a non-gaming machine in the home.
The IBM-compatible PC was the focus of innovation from many different companies. It wasn't a one-vendor platform like the Amiga. That is the ultimate cause of its demise. If the fate of the PC had stayed tied to IBM, it would have suffered the same end. Apple had a similar problem with the Macintosh in the 1990s and it almost killed them. If Apple's stewardship ever wavers, that is sure to happen again. It will take a lot more before it can become an immortal (undead?) platform like the PC.