I am not so sure about that.
It seems that duality in string theory relies basically on large-N limit considerations. So perhaps it's exactly the other way round: not ordinary gauge fields are (low-energy) limits of strings, instead string theory may be the large-N limit auf certain gauge theories. If this is true, SUGRA may very well be an own candidate w/o the requirements to derive it from strings.
One question: which SUGRAs are the limits of certain string theory and which are not?