At that time compatibility was not an important aspect. For example only almost 2 years passed between the release of C64 and the Plus/4, yet the machines repre

At that time compatibility was not an important aspect. For example only almost 2 years passed between the release of C64 and the Plus/4, yet the machines represent different worlds without any compatibility, and we are not even talking about the VIC-20 and the C16. The C128 had a 99% compatible C64 mode but in turn had a more limited number of SW available (that's the other side of the story - if you provide compatibility, old software will be used and maintained instead of starting new developments, and if no new software is developed for a platform it's dead). These systems were anything but cheap, and once you have bought a computer, you'd locked yourself to the platform.