what would the situation have been had Ascension Island been unavailable for a diversion?
The answer is to have diverted somewhere else. The B777LR is certified up to (I believe, don't know whether Delta's birds have gained this) 330 minute ETOPS. This provides a huge radius for landing. Look at the map in the following link
Great Circle Mapper. Ascension Island is the dot with (@410 knot) 120, 180, 240 and 330 minute circles around it in the middle of the South Atlantic. The worst case scenario would have been a failure right over where ASI is located, but with no airport at that location. As you can see, many other airports are within the diversion range.