It might be fun to do the math on this.
I don't pop-rush much, but would a 59 shield granary cost 3 citizens to rush?
What type of set up would overcome a granary pop-rush? I think one of two possible starts would make it feasible - a high food start or a low shield start (hypothesis). High food would replace the lost citizens quickly (especially with a granary) mitigating the loss, low shield would give each sacrificed citizen an adjusted higher value (from a spt viewpoint). If you have both high food and low shield, that would be the best possible set up?
I suppose there are a lot of factors to consider - such as would it better to rush a settler to build the next city rather than killing 3 citizens to make a granary (assuming you have already decided to rush, period).