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It is about treating new versions with major API breakage as if they were completely new libraries, not as a "maybe, possibly drop-in replacement, iff we use just the right feature subset". E.g. RxJava changed their maven artifact ('io.reactivex:rxjava:1.y.z' -> 'io.reactivex.rxjava2:rxjava:2.y.z') and the package name where in which their code lives ('rx' -> 'io.reactivex'). This makes it possible for both versions to be used at the same time while transitioning, without breaking dependencies that rely on the older version and without having to bother with automatic multi-versioning in buildtools.

With that in place it is questionable what the actual advantage of semver vs a simpler monotonous versioning schema (like build number or UTC date) is.