Heaven and Earth, as we know it, shall pass away into a New Heavens and a New Earth, but God's words, plural, indicating the importance of plenary inspiration and preservation of "each of God's inerrant, infallible words, will last for eternity. The pure and uncorruptible words God breathed out, and his writers breathed in, are preserved for us today, and we will always have them because they come from the library of heaven.
I agree that this is part of the reason. But another part of the reason is that Jesus was saying that not even one single item of what He was saying would fail to come to pass. This is a common theme in the prophetic scriptures.
We read, for instance, in Isaiah 34:16, "Seek ye out of the book of the LORD, and read: no one of these shall fail, none shall want her mate: for my mouth it hath commanded, and his spirit it hath gathered them." And in Isaiah 46:9-11 we read, "Remember the former things of old: for I
am God, and
there is none else;
I am God, and
there is none like me, Declaring the end from the beginning, and from ancient times
the things that are not
yet done, saying, My counsel shall stand, and I will do all my pleasure: Calling a ravenous bird from the east, the man that executeth my counsel from a far country: yea, I have spoken
it, I will also bring it to pass; I have purposed
it, I will also do it." And in Jeremiah 4:28 we read, "I have spoken
it, I have purposed
it, and will not repent, neither will I turn back from it." And in Ezekiel 12:25 we read, "For I
am the LORD: I will speak, and the word that I shall speak shall come to pass;" ANd just three verses later, in Ezekiel 12 :28 we read, "the word which I have spoken shall be done, saith the Lord GOD." And in Ezekiel 22:14 we read, "I the LORD have spoken
it, and will do
it." And in Ezekiel 24:14 we read, "I the LORD have spoken
it: it shall come to pass, and I will do
it; I will not go back, neither will I spare, neither will I repent." ANd in Ezekiel 36:36 we read, "I the Lord have spoken
it, and I will do
it."
This is also why Jesus said "Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled." (Mathew 5:18) The jot and tittle were not even letters of the Hebrew alphabet. They were marks used to make up the individual letters of the Hebrew alphabet. Jesus was saying that not even a part of a single letter of the law (the books of Moses) would fail. Everything would be fulfilled exactly and precisely, down to the tiniest detail.
That is why the scriptures so often repeat the formula, "that it might be fulfilled which was spoken by..." These words, or something very similar to them, are repeated 22 times in the scriptures in regard to a literal fulfillment of an explicitly stated prophecy. 21 of these 22 times are in the four gospels in regard to explicitly stated prophecies fulfilled literally by Jesus. The other one is in Ezra 1:1, but it is also in regard to a literal fulfillment of an explicitly stated prophecy.