We could have both by introducing another keyword than "default" here.

In some cases you want "default" handling, in other cases it's a runtime detection of a code issue. E.g. you could add a "fallback" case which is the same as "default" yet if your code doesn't exhaustively handle all sealed class cases won't compile. This would still create a fallback, if your sealed class is widened later on, but still allow you to error out if it's already the case right now.