What was the significance of the promise that Avraham "would join his forefathers in peace"?
Rashi: HaSh-m was telling him that he would live a peaceful life, and that he would not suffer any of the travails which He had said earlier his children would suffer. 1
Ramban: HaSh-m was telling Avraham that it was the aspects of slavery and subservience that he was not destined to suffer. We find that he did in fact experience being a stranger (Bereishis 21:34). 2
Gur Aryeh: Otherwise, why mention this here? Rather, the intent is the servitude would not begin in Avraham's lifetime.
See R. Chavel's notes on the Ramban; and see Mizrachi. Gur Aryeh explains - Avraham did not suffer any degradation or oppression in being a stranger, for all acknowledged him as "a prince of G-d" (23:6). Also refer to 15:13:4.4.
Seeing as Terach was an idolater, how could HaSh-m promise Avraham that he would join him after he died?
Rashi: This teaches us that Terach did Teshuvah before he died. 1
Also refer to 11:32:3, and refer to 11:27:151:1.
What was the significance of "the good old age" that Hash-m refers to?
Rashi: This was an assurance, a) that Yishmael would Teshuvah in his lifetime; and b) that Esav would not sin publicly as long as he (Avraham) was alive. Indeed, Avraham died five years before his time, so that he should not see that happen, and Esav sinned on the day that his grandfather died.
