Posting for someone else:
The Rashbam on Bereshis 22:1 brings that Hashem, in retaliation for Avraham giving 7 sheep to Avimelech, says that Avimelech's descendants will destroy 7 sanctuaries of Avraham's descendants; the mishkan, Givon, Nov, Shiloh, Gilgal and the First and Second Temples. How is this possible, given that the First and Second Temples were destroyed by the Babylonians and Romans respectively, and they were not descendants of Avimelech, who was a Philistine?
The Rashbam is citing Midrash Shemuel 12.1. Midrash Shemuel is a later work (first cited by Rashi) which collates earlier rabbinic material (such as Bereshith Rabbah). So really the question ought be on the text as transmitted by the Midrash Shemuel. In the chapter on Midrash Shemuel in Ginze Midrash (by Zvi Meir Rabinowitz) p. 187 fn. 58 he writes:
שני בתי המקדש הראשון והשני לא נחרבו ע"י הפלשתים אבל תולה חורבנם בחטאו של אברהם שכרת ברית עם אבימלך. בב"ר נד ד עמ' 580 חסרה המלה 'בניו' ומובא 'כנגדו מחריבין מבניך'
The First and Second Temples were not destroyed by the Philistines, however their destruction was contingent upon the sin of Abraham who forged a covenant with Abimelekh. In Gen. Rabbah 54 4, p. 580, the word "his sons" is absent, and brought in its stead are the words "opposite him your sons will be destroyed".
So it would appear that in earlier iterations, such as in Bereshith Rabbah, it is not suggested that the Philistines are descended from Abimelekh. Rather that the Midrash Shemuel introduced an error in the transmission of this midrash from Bereshith Rabbah, and that the Rashbam merely preserved the text as he received it.
