Calvinism does not teach “once depraved, always depraved.” It is “once saved, always saved,” in other words, you can’t lose your salvation.
If it was “once depraved, always depraved” no one could be saved.
Good grief, it’s a joke. Surely there’s something in the Institutes about having a sense of humor.
But anyway, you’re wrong: Calvinism teaches that there is no moment of salvation, only a previous moment of predestination, so anyone not among the elect really is “always depraved.”