Commodore had already failed in 1984 with an incompatible model with the Plus/4 (which my parents got me for Christmas as my first computer, since they didn't know any better and the Plus/4 was the "new" thing).
The Plus/4 at least took the same peripherals as the C64, so I was able to buy a C64 a few years later and preserve much of the investment. The Plus/4 wasn't bad, but its graphics and sound were worse than the C64. It had an office suite bundled in ROM that wasn't very well-regarded.