A few points which tend to downplay the srgument posed is that Scripture still existed prior to the Gospel. The Torah was well written and recieved within the Jewish community and early Church.
The Holy Spirit provided many gifts to the early Church, and the gift of Pastor-teacher was among them. Divine revelation was also available and simply because an anthology we name 'the Bible' wasn't immediately available, doesn't imply God didn;t have a purpose and plan for every believer's life predetermined and planned in eternity past. It also didn't preclude Him from revealing His will by His grace to believers at that time.
A great perspective is to consider how much more blessed we are today to have such a wealth of divine revelation made so easily available to us to study through faith in Him.
You are correct .. progressive revelation has been the key.