It wouldn't have been just an NES chip. It would've had to also include a separate PPU (in addition to the two already in the SNES), a NES cartridge I/O slot, a whole different video out architecture (the NES didn't support composite out), and maybe more. Those are just the ones I know for sure.

Besides, the SNES was already going to cost significantly more than the Genesis. They were wary of widening that price gap still further when the owners of the older system still owned the older system and could easily plug it back in. Further, they were launching the SNES in North America with five launch titles and eight more on deck over the following month, with a total of thirty games coming out before that Christmas. I don't think they were worried about having enough content for people to play on that new system.