Even in the computer world, backwards compatibility wasn't always guaranteed. The Commodore 64 wasn't really backwards compatible with the VIC-20 though the Commodore 128 was compatible with the Commodore 64. I think the Apple IIgs was the only 16-bit computer that was backwards compatible with it's 8-bit counterpart. Once into the 16-bit (and DOS) era, backwards compatibility was a virtual guarantee...for the most part. But even today, software compatible with older operating systems are being left behind...outside of emulation.