As somebody that has done a fair amount of usability testing, that much of why people think redesigns are unusable are simply because they were used to the old version and haven't yet spent much time with the new version. But if you show the new design and the old design to somebody that has used neither, they frequently prefer the new design. And over time, everybody kind of likes (or at least gets used to) the new design.

That doesn't mean that all redesigns are necessarily good, or even that redesigns are necessary, but the kneejerk "it's unusable!!!!!" is so cliched.