Meaning of "And you shall serve your brother" specifically for Esav and Yaakov

Question

Unlike another question that asks about the vague prophecy Rivkah was given about her twins, I ask specifically about Itzhak's statement to Esav:

... וְאֶת־אָחִיךָ תַּעֲבֹד ...׃ - And you shall serve your brother; Genesis.27.40

As I can see, most commentators skip this part of Isaac's prophecy. Unlike the other question, this verse has a proper grammatical form "את אחיך" similar to "אֶת ה' אֱלֹהֶיךָ תִּירָא אֹתוֹ תַעֲבֹד", meaning serving and worshipping, this verse states clearly that Esav should subserve to Yaakov.

Was it really a blessing or a prophecy? If a blessing, what kind of blessing is that? If a prophecy, what does this mean for Esav and when was it fulfilled?

Answer 1

Rav Hirsch says that as long as you (Eisav) insist on physical mastery of the world, then he will only come to lay it down at the feet of Yaakov. It is only when he understands the spiritual requirements of man will he understand what it means to be part of the family of Abraham.

As long as you persist in ruling, looking on and using every material conquest only as the step for further conquests, for so long you are in reality working for your brother (only proving to the world and ultimately to yourself, the futility and lack of security which the power of the sword really acquires I.L.). As long as you are רב, for so long will רב יעבד צעיר, only כאשר תריד when you com down by your own free will, voluntarily acknowledge and submit yourself to the ideals of your brother, will you emancipate yourself and stand on an equal footing to your brother

Answer 2

The full pasuk is important for context:

וַיַּ֛עַן יִצְחָ֥ק אָבִ֖יו וַיֹּ֣אמֶר אֵלָ֑יו הִנֵּ֞ה מִשְׁמַנֵּ֤י הָאָ֙רֶץ֙ יִהְיֶ֣ה מֽוֹשָׁבֶ֔ךָ וּמִטַּ֥ל הַשָּׁמַ֖יִם מֵעָֽל׃ וְעַל־חַרְבְּךָ֣ תִֽחְיֶ֔ה וְאֶת־אָחִ֖יךָ תַּעֲבֹ֑ד וְהָיָה֙ כַּאֲשֶׁ֣ר תָּרִ֔יד וּפָרַקְתָּ֥ עֻלּ֖וֹ מֵעַ֥ל צַוָּארֶֽךָ׃

And his father Isaac answered, saying to him, “See, your abode shall enjoy the fat of the earth and the dew of heaven above. Yet by your sword you shall live, and you shall serve your brother; but when you grow restive, you shall break his yoke from your neck.”

The Seforno says:

ופרקת עלו מעל צוארך. כי אמנם תהיה מלומד מלחמה ומוכן לגדולות אבל אם בשעובדך תפנה אל עבודת האדמה ושאר המלאכות לא תהיה חפשי לעולם משעבוד אחיך או משעבוד זולתו:

...Because indeed you will be a scholar of war and ready for greatness, but if in your work (בשעובדך) you turn to the work of the land and the other crafts, you will never be free from the servitude of your brother or the servitude of others.