"One major reason is sprites: Amiga can display 8 4-color sprites, or 4 16-color sprites, and the colors are shared with the bitplanes."

Per scanline. However, you can multiplex them with copper to your hearts content.

"SNES can display 128 16-color sprites, and the sprites get 8 palettes all to themselves."

32 per scanline, but then almost all of those sprites would need to be 8x8. Just 17 of 16 pixels wide sprites.