[kaba0]

I have to agree with you. On the other hand, closed hierarchies can be an elegant solution to certain problems. Eg. sealed classes (and basically their single-class counterparts, final classes) avoid the mentioned problems in Java’s parlance.

One exception comes to mind though — Java’s SAMs, or in the general case, classes that more or less only wrap around a few methods intended to be overridden/implemented with clear requirements (but maybe this use case also should be restricted to interfaces?) But the default should be to add an explicit open instead of defaulting to non-final.