Don't recall anyone saying Benedict wasn't up to the task of being pope, so that's a new one.
Look, he is now one of the oldest men ever to be a pope, at 85, and although in this modern world people live longer lives, when someone is 85 and suffering one health issue after another finally says he just can't do a demanding job effectively anymore, his comment ought to be met with understanding ears. All long and successful histories have many failings, that others with long and less well documented histories with thereby few documented failings, can and do use to assert anything they want. That's wrong. So just be nice to each other, kids.