Bereshit 28:15 has G-d telling Jacob that He will not forsake him until He fulfills what He promised to Abraham. Doesn’t this seem to imply G-d will leave us after that? Do any commentators say anything?
It is not a statement implying G-d will leave us at a certain point - chas veshalom.
See the Rashbam who explains this is just a reference to Yaakov journeying, and therefore Hashem was promising that he will guide Yaakov with special protection and not abandon him.
The Tzror HaMor takes a typically mystical approach explaining that after Yaakov saw the ladder placed on the ground and connecting all the worlds, and Hashem as it were stands over them and binds them together by the power of the souls of the righteous, it is impossible that Hashem will ever abandon us.
As it says in Yirmiyahu 46:28
אַ֠תָּה אַל־תִּירָ֞א עַבְדִּ֤י יַֽעֲקֹב֙ נְאֻם־יְהוָ֔ה כִּ֥י אִתְּךָ֖ אָ֑נִי כִּי֩ אֶעֱשֶׂ֨ה כָלָ֜ה בְּכָֽל־הַגּוֹיִ֣ם ׀ אֲשֶׁ֧ר הִדַּחְתִּ֣יךָ שָׁ֗מָּה וְאֹֽתְךָ֙ לֹא־אֶעֱשֶׂ֣ה כָלָ֔ה וְיִסַּרְתִּ֙יךָ֙ לַמִּשְׁפָּ֔ט וְנַקֵּ֖ה לֹ֥א אֲנַקֶּֽךָּ׃
But you, have no fear, My servant Jacob — declares G-d — For I am with you. I will make an end of all the nations among which I have banished you, but I will not make an end of you! I will not leave you unpunished, but I will chastise you in measure.