God's Promises -- Are They Always Kept?

There is a very famous Midrash on this week's parsha. When Yishmael and Hagar are dismissed from Avraham's house, they end up in the wilderness where the boy begins to die of thirst. As God was preparing to show Hagar where the well was (to save Yishmael's life), the angles angels protested and petitioned God to let Yishmael die. They said that in the future, the descendants of Yishmael will cause many Jews to die of thirst and, to prevent this from happening, God should let Yishmael die now. God, however, responds by saying that He is judging Yishamel as is he is now (ba'asher hu shom) and since, at the present moment, he is not deservant of death, he is going to be saved.

It's a very nice Midrash, one that gives us some insight into how God administers Divine Justice. However, I had a very simple question. How could the angles angels petition God to let Yishmael die? Didn't God explicitly promise both Avraham and Hagar that Yishmael would grow up to found a new nation? Wouldn't God's promise to Avraham and Hagar preempt any possibility of listening to an angelic plea to let Yishmael die?

I spoke about with with a fellow congregant this weekend. He informed me of a Tzlach (which I did not see inside) which discusses how God's promise can be broken if it's for the benefit of the k'lal.

I'm not so sure that I really agree with that reasoning. There is a gemara (the exact location of which escapes me at the moment) which states that when God makes a conditional positive promise, the promise is always kept, even when the condition is not kept. It is logical to state, that if God always keeps a promise, even when He has a "legal out" (i.e. when the condition is not kept), could it not be said that He always keeps a positive promise when it is absolute and unconditional and therefore does not have a "legal out?"

I explained this to my friend on Shabbos, and gave him an example of his reasoning taken to the ultimate degree. If you are going to say that even God's absolute promises are subject to revocation "for the good of the K'lal), then suppose, under some bizzarre set of circumstances, it's beneficial for the K'lal that God should revoke His promise to Noach. Does that mean that He will then flood the world again despite His promise? That just does not sound right to me.

So, going with the assumption that God's positive promises are always kept (especially when given unconditionally), why did the angles angels even bother arguing? Furthermore, why did God give the argument of ba'asher hu shom? Why not simply state that He was bound by His promises to Avraham and Hagar?

The Wolf