Jamieson, Fausset and Brown on Hebrews 8:13

13. he--God.

made ... old--"hath (at the time of speaking the prophecy) antiquated the first covenant." From the time of God's mention of a NEW covenant (since God's words are all realities) the first covenant might be regarded as ever dwindling away, until its complete abolition on the actual introduction of the Gospel. Both covenants cannot exist side by side. Mark how verbal inspiration is proved in Paul's argument turning wholly on the one word "NEW" (covenant), occurring but once in the Old Testament.

that which decayeth--Greek, "that which is being antiquated," namely, at the time when Jeremiah spake. For in Paul's time, according to his view, the new had absolutely set aside the old covenant. The Greek for (Kaine) New (Testament) implies that it is of a different kind and supersedes the old: not merely recent (Greek, "nea"). Compare Ho 3:4, 5.