Did’ja all catch that they knew the instrumentation signature of burn-through and “that is one of the things we always watched on re-entry anyhow.”
Yes, I sure did. Knowledge of all possible contingencies doesn't predict their occurrence. Yet it still had to have been for even a Top-Gun Graduate/Navy Test Pilot such as Commander Gibson, one extremely white-knuckled re-entry.
If they had not successfully re-entered and landed at Edwards AFB, the entire crew would have been lost; the Shuttle would have been impossible to retrieve from the Pacific Ocean; no one would have known about the extensive damage to Atlantis' heat tiles; and Atlantis being the second Shuttle mission following the Shuttle Challenger disaster, without a doubt the entire Space Shuttle program would have ended.