A surviving kingdom of Jerusalem needs Egypt in Christian hands.
Probably. But this objective was unreachable : crusader expeditions were too limited in men and ressources to really hold control of Upper Egypt.
IMHO the 4th crusade may not have been the best organised one (as if any crusade was really well organised) but it has a reasonable chance to get the delta of the Nile, with Alexandria and Damietta.
To get into the Delta was possible, indeed. Controlling it, conquering it and holding it...
More organized and planned (yes there were, you can generally see them at their, even if limited, success) failed to do so.
And 4th Crusade was...well, let's say context didn't helped at all.
- War in France, Germany and England. A good part of the traditional recruiting ground was unavaible. (Admittedly, they still managed to gather, something around 20 000 men)
- No real financial support, making them obligated to Venice (that was determined to at least use them in Adriatic, hence plunder of Zara and therefore half of the party just giving it up, and pope excommunicating the whole expedition)
Crusaders had quite a stroke of luck, having a pretext and a limited support thanks to Byzantine inner conflicts, allowing them to stay in Byzantium before just sacking the city.
10 000 men, going directly in Egypt while excommunicated (without forseeable reinforcements) aren't going to do much on themselves, directly attacking and attacked by Ayyubids. A threat, granted, but not likely to conquer parts of Egypt.