I think backwards compatibility would have been tougher to do with cartridges than discs for various reasons. I know they did this with the Game Boy line, but those cartridges were designed differently and the hardware was similar, whereas the N64 hardware was totally different from the SNES, and the SNES used a lot of expansion chips in its cartridges.
Assuming it was possible though, I don't think it would have mattered as I don't think backwards compatibility has ever been a selling point for a console. Take the PS2 for example, I think people just saw it as a bonus feature, and what was really selling that system were the improvement in graphics and the DVD playback. BC by itself wouldn't have moved PS2 systems... just like how today, the Wii U is the only current console with BC and it's selling the worst of the three. BC won't save a flawed system, but makes a good bullet point on a desired system.
I don't think most people care about having more than one system connected to their TV, at least not enough to be a selling point, but I think it could matter more to handhelds. People don't want to carry more than one device so having a system that can play both new and old games could be enticing. Heck, I know that BC is definitely a feature that pushed me to buy a 3DS since I didn't have a DS, and I have way more DS games than 3DS. So I think BC may have helped sell systems like the GBA, DS, and 3DS, especially in the early days.