True, but they're not on the same level of difficulty as superbosses. They (Plague, Wyvern, etc.) seemed to me to be similar in difficulty to, say, Necrophobe in FFV. I'll grant that they're arguably harder, but the idea of making an FFV reference was to imply the difference in difficulty between Necrophobe and Omega/Shinryuu.

They are part of the motif and tradition of bosses/powerful monsters guarding treasure chests; of that there's no doubt. I apologize if I misinterpreted your comment.