We should not overlook the role of technological advances and their effect on notions of canonicity. In specific, the invention and proliferation of the codex (a.k.a. book) that displaced the scroll had a tremendous impact on what canon means and thereby on whether questions about its being open or closed even make sense.
Up to and even during the New Testament period, the various Scriptures were kept each in its own scroll. The typical Jewish classification of the Scriptures, with the Law as the most central, through the Prophets, and out to the Writings as the most peripheral (and then the various texts now termed the Talmud beyond that) makes sense in that context. These texts were a plurality. There wasn’t a clear demarcation line between canonical and non-canonical.
The codex allowed multiple texts to be compiled into a single collection. This raised the question of what should be included and what should not. Rather than the concentric circles model of canonicity that was facilitated by the plurality of scrolls, the codex by its very nature required all the texts to be either in or out of the official canonical collection. The texts thus became a unit, the Bible. Everything outside the Bible came to be seen as simply uninspired and everything in it as simply inspired.
In this new situation, the concept of “canon” inevitably took on a new form. In the concentric circles model of canonicity the concept of “open vs. closed” never arose because canonicity was a matter of relationship between texts. Now, however, canonicity became a status that certain texts had. Being canon defined only the relationship between the Bible and everything outside of it, and no longer applied to the internal relationship between its constituent texts.
I suppose some might find this disturbing, since it could be viewed as unsettling our commonly held understanding of canonicity. I’m not convinced that we should see it that way, though. There’s no reason to say that this development wasn’t also guided by the Holy Spirit. The knee-jerk assumption that all post-1st c. developments are uninspired, which is (ironically enough) abetted by the in-or-out understanding of canonicity, is incorrect. This development is arguably a positive one. For my part, I’m inclined to think that both ways of thinking about canonicity are useful and both are limited. What is needful is wisdom to know which way of thinking about canonicity is most appropriate in any given circumstance.