If the _stub package does not exist in the older catalog, we must not drop the _stub from the newer catalog. If we did that, we would break the upgrade path. The upgrade looks like this:

N-2 CSWoldname/oldname
N-1 CSWoldname/oldname_stub → CSWnewname/newname
N   CSWnewname/newname (_stub is removed)

Imagine that you have CSWoldname instsalled on a system and are subscribed to N-2. Then you drop oldname_stub from N-1 and you upgrade your system from N-2 to N-1. The CSWnewname package would not be pulled in, right?