When the angel finally calls from heaven, the Targum of Pseudo-Jonathan on Genesis 21:17 gives the reason out loud: for the righteousness' sake of Abraham. Ishmael lives not because he earned it, and not even entirely because of his mother's tears, but because his father's zechut — the merit of Abraham — reaches further than anyone standing beside him can see.
The Aramaic goes further. Neither shall judgment be according to the evil which he will do, but according to the righteousness of Abraham is mercy upon him in the place where he is. The Targum of Pseudo-Jonathan is answering a question that had already begun to trouble the early rabbis: what about Ishmael's future? What about the descendants who will oppose Israel?
The Aramaic resolves it on a single Hebrew phrase from the verse — ba'asher hu sham, in the place where he is. At this moment, in this wilderness, this child is innocent. He is judged by his present, not his future, and covered by the merit of the father who prayed for kingdoms.
The Maggidim taught that this line is the Torah's quiet argument against predestination. The takeaway: heaven judges the person standing before it, not the person they may become. Right now is what counts.