I’m convinced that Paul had no idea that some large and far-future Christendom would hold his own writings as equal to, or even higher rather, than the “law and prophets” that were already held sacred in his day. For one thing, nobody in the early church thought in terms of some far-future establishment because they were convinced Jesus was returning almost within their own life times.
But another reason I think that is because of Paul’s letters themselves. They have his personality oozing out everywhere. He writes in a personable manner that I doubt anyone would choose to use if they thought they were setting down sacred writ that others would be memorizing and reciting down the ages. I mean … how many of us would “boast like a fool” (freely admitting that!) to drive home a point that our own ministry is not inferior to that of our colleagues? Or let our passionate flash-point anger / exuberance show through so freely on so many lines? We would probably clean a lot of that up --get it all polished up if we thought myriads of future strangers would be memorizing and poring over our every word. So, ironically, I think it is the personable nature of his letters – merely meant for distribution just among some local churches in whom he had personal investment and addressing their particular problems that make his letters so powerful and valuable; the very ‘clay jars’ through which the Spirit shines, to borrow one of Paul’s metaphors.
It is true that it didn’t take long before his writings were recognized widely for their value, as Peter refers to them approvingly in one of his letters.
So when Paul writes in 2nd Timothy about all Scripture being inspired, I highly doubt he was elevating his own letters (writing those very words) to be on par with Moses and the prophets. But note that even if my speculation on this is true, it does NOT mean that we are wrong to consider his letters as part of that today. The Spirit teaches us as well, and we can creatively engage the Scriptures as Paul and the Apostles did back then with our own expanded Scriptural traditions as well. Wrestling with God (the very meaning of the name “Israel”) and wrestling with scriptures too (the full canons recognized by our various traditions today) is not the same as dishonoring the same. Quite far from it, in fact. I fully agree with Enns on that. We follow Jesus’ example in doing so. “You have heard it said … but I say to you …” was not in the least bit dishonoring to the law. On the contrary, we have it on good authority that it is the fulfillment of it.