Ishmael was sent into the wilderness with his mother,this would resemble the faith of Ishmael and his descendants in the days to come. Issac's descendants would be more spoiled, and they   would fall into sinful traps easier. GOD'S final plan is for Ishamel  to be sent to Issac in the latter days to show  him how far he has fallen. Ishmael will act as a mirror to Issac's descendants. Sent into the wilderness, only to return to Issac to reject him again, but eventually   Issac will have to  receive Ishmael. Everything in GOD'S kingdom plan works for a reason. Everything is dualistic in GOD'S universe. Issac will have the physical blessings while Ishmael would have the spiritual blessings. The older half brother is wiser than the younger beloved brother. We see this story all throughout the bible. The older brother has wisdom, while the younger brother gets the glory. It takes a balanced combination of the two nations to bring GOD'S kingdom here on earth.