I'm not having it both ways. I'm cutting off the objection you could raise on behalf of the completeness of Scripture, that St. John was engaging in hyperbole, and we should know this from his 'I suppose'.
St. John's conclusion shows that as a record of Our Lord's deeds, Scripture is incomplete. (Unless one hold that verse to be false, or open up a big problem for sola scriptura hermeneutics by allowing rhetorical devices like hyperbole into the text.)
So which is it? Does Scripture contain a falsehood, or is it not a complete record? If the latter, and I trust you prefer the latter--to return to the original point of this thread--if incomplete in its record of Our Lord's doings, which are its main focus, why should there be an objection to it being incomplete as regards the details of the life of the Virgin Mary?
It doesn't have to be a "complete" record in order to be complete scriptures. Jesus went to the bathroom, maybe there is a good reason that the Holy Spirit didn't write about it. Just maybe it isn't important to the edification of the Church.
It doesn't have to be a "complete" record in order to be complete scriptures. Jesus went to the bathroom, maybe there is a good reason that the Holy Spirit didn't write about it. Just maybe it isn't important to the edification of the Church.