@Gmsod,
The question comes down to what Jesus told His Church to do, not what the state of extant Scripture was in the 1st century A.D. – after all, back then, He was getting on them for not understanding what God had intended all along, as recorded in the OT.
The claim that you’re addressing – namely, that the Bible teaches plainly and clearly – has two motivations: first, from the perspective of Sola Scriptura, the claim is that no teaching source but the text itself is necessary. Second, from the perspective of Protestant theology, the claim is that no authoritative teaching body is necessary.
To address these claims, we need to look at what Jesus instituted and set up for the creation and formation of disciples for His Church.
To answer the first question, we have to ask “what teaching method did Jesus tell His Church to use?” The answer is found in the Great Commission, “make disciples of all nations… teaching them to observe all that I have commanded you.” (Mt 28:19) But… what method of teaching did Jesus have in mind? Mark and Luke tell us that Jesus’ intent was proclaiming and preaching, respectively (Mk 16:15, Lk 24:47). In Mark 16:15, the word used is κηρύξατε, and in Luke 24:47 it’s κηρυχθῆναι, both of which are based on the same root word, meaning “proclaim” or herald" or “preach publicly”.
Even Paul says “Christ sent me to preach the gospel” (1 Cor 1:17), and that he was called to deliver a “speech and message… in demonstration of the Spirit and of power” (1 Cor 2:4). He explicitly makes the point that this teaching is through oral preaching and aural reception of the gospel: “how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching?” (Romans 10:14)
Now, I don’t mean to say by this that the Bible is counter to Christ’s intent. Clearly, it is the Word of God, given to us by God to learn His will for us. However, the intent is clear: it is preaching – by mouth, for the ears of listeners – through which the gospel is meant to be learned. Not merely reading, but hearing others preach. That, in itself, would seem to discount the claim that the Scriptures are meant to be the sole source of receiving the gospel message. Are they “profitable for teaching”? Absolutely! Are the the sole means? Not even according to the Bible itself!
But, what about the question of an authority who teaches? We see the answer to that question in Matthew 16 and Matthew 18. Authority is given first to Peter, and then to all the apostles. Therefore, teaching authority is given to persons, not to a book. So, in terms of authority, we see that assigning it to a book runs counter to Christ’s intent.
So, on both counts, I think I would respond, “no – Christ commands us to hear and obey authoritative teaching, not to read a book and presume it can be interpreted by just anyone.”
Hope this helps!