What did the fact that Avraham had no children have to do with the fact that Eliezer would inherit him?
Rashi: What satisfaction can one feel in the knowledge that all his vast wealth will land up in the hands of his slave, and not his son?
Seforno: Avraham was pointing out that, even if Hashem would grant him a son, by the time that son would grow up, Eliezer would have already made Avraham's property his own, and the son would be too small to wrest it from his hands. 1
Avraham already said that he is childless in the preceding verse (15:2). Why did he repeat this?
Moshav Zekenim: Avraham did not say this, rather, Moshe. In other words, the Torah narrates Avraham's complaint, and how Hashem answered.
How could Avraham complain that he is childless? Hashem had already promised to grant him children (12:7)!
Riva #1: He was concerned lest sin cause that the promise would not be fulfilled.
Riva #2: Avraham pointed out that even if he will have a son now at the end of his life, after Avraham dies, Eliezer will seize the estate. 1
Hadar Zekenim: He was concerned lest Eliezer kill the heir.
Hashem had already assured Avraham of children in Chapter 12, yet Avraham still worried that some sin on his part would cancel that assurance. But why did he worry? The Gemara (Berachos 7a) states that Hashem does not renege on His word to bestow good?
Maharal #1 (Gevuros Hashem Ch. 7, p. 40, citing Rambam (Intro. to Peirush Mishnayos)): The Gemara refers only to a Navi who prophesied good upon others. That good will always come about, lest the Navi be accused of fabrication. For an assurance relevant to the prophet himself, however, there is no guarantee for the righteous.
Maharal #2 (ibid, citing Radak): Sin can partially affect the fulfillment of a prophecy, just not completely. 1
Maharal #3 (ibid.): There is a difference between an assurance (Havtachah), and a prophecy (Nevu'ah). 2 An assurance can be changed because it is dependent on the recipient, whereas a prophecy is independently true and will be fulfilled. 3 This is why prophecy is usually related in the past tense, it is already a reality.
The Maharal questions the approaches of the Rambam and Radak.
A Havtachah relates to an individual, whereas a Nevu'ah relates to the Tzibur. See also Gur Aryeh to Bereishis 32:8, who writes about this at length.
A prophecy of misfortune is different, however; it is dependent on the recipient. Hashem bestows only good; the point of such a prophecy is to bring us to Teshuvah, in which case the dire prophecy is cancelled.
QUESTIONS ON RASHI
Rashi writes: "Behold (Hein) - [Avraham asked,] of what use is all that You would give me?" Why interpret the verse in this way?
Gur Aryeh: The word "Hein" denotes a response (e.g. Bereishis 44:8). Avraham was still responding to Hashem's words in 15:1.