No, we haven't.
You have a point there. We are Christians--or, speaking for myself, I am a Christian--
because I believe the Bible. It is possible not to believe the Bible and not to be a Christian. Millions of the worlds Hindus and Buddhists, for example, do not. For them the question about Sola Scriptura is irrelevant.
Here, however, we supposedly are having a discussion between Christians about the Christian religion, between people who believe that the Bible IS indeed the revealed word of God. The question must be, therefore, if it is an adequate quide to Christian doctrine. If a Hindu were to come onto this forum, against the CF rules, it would probably be useless to argue whether or not the Bible is worth believing.