Abraham had asked for Ishmael to be the heir of the promise (Genesis 17:18). Targum Pseudo-Jonathan on Genesis 17:19 preserves the Lord's answer, and it is not what Abraham requested.
In truth — be-kushta, in real and fixed reality — Sarah thy wife shall bear thee a son. And thou shalt call his name Izhak, Isaac. And with him I will confirm My covenant for an everlasting covenant to his sons after him.
Three words do a great deal of theological work. In truth — this is not metaphor; Sarah will carry a child. His name — Isaac, the laugher, the son whose conception begins as a father's disbelieving chuckle (Genesis 17:17). With him — the covenant established at such cost in this chapter will run through Isaac, not through Ishmael, and not through any future son.
The Targum is careful not to erase Ishmael. The next verse will bless him with twelve princes and a great nation (Genesis 17:20). But it draws the line of the covenant clearly. Blessing is wide. Covenant is specific. Ishmael gets a future. Isaac gets a Torah-future.
The Maggid hears how painful this must have been for Abraham. He has lived thirteen years loving the son he has. He is now told, gently and firmly, that the promise he has been waiting for is still a year away, and that it belongs to the child not yet conceived (Genesis 17:19). Sometimes the answer to a father's best prayer is: I have something else in mind. The Lord does not argue with Abraham's love for Ishmael. He simply widens it, and keeps the covenant on its own path.