I don't think this was really a practical thing and I think there'd be other signs of an attempt at if if so. The final version of the SNES PPU probably would h

I don't think this was really a practical thing and I think there'd be other signs of an attempt at if if so. The final version of the SNES PPU probably would have resembled the NES PPU more, for example, in small ways like tile sizes at the very least. There's at least one mode on the snes ppu that's kiiiinda like the nes ppu's standard behaviour but it's just off enough to make reusing code for it fairly hard I think. And then I think you'd have seen a mapper for the NES that more closely resembled the 65816's native addressing, since that would definitely have eased such porting efforts. But even the MMC5, which was first used the same year the SNES came out didn't move much in that direction. Tbh I think you're also overestimating the assemblers they used at the time as well, both in terms of the expressiveness of the macros they had available and and kind of dead code removal you might be imagining happening. I think a much simpler explanation is just that they had it in mind as a possibility from the start and they made some design choices that were both easy and would have allowed it. They may have been leaving open the possibility of an adapter cart like the genesis power base converter (which had much less work to do) too. But they also decided fairly early on that it wasn't really worth it economically and nixed it early enough that it didn't make a huge impact on the architecture.