Why does it say "Ein Ketz l'Chol ha'Am"?
Rav Sadya Gaon: My thoughts encompassed all of them.
Rashi: They multiplied excessively - "Zar'am Nachon Lifneihem" (Iyov 21:8). A woman become pregnant and gave birth in three days. It says "Yeshalechu cha'Tzon Avileihem" (ibid. 11); here it says Nachon, like "v'Hayu Nechonim la'Yom ha'Shelishi" (Shemos 19:11).
Seforno: Most people make temporary life their primary objective. They abandon the first youth and his Chachmah (refer to 4:15:3:5).
Metzudas David: In the reign of the old king, and of the first youth, there was not a limit to the good that they had [that would satisfy them]; they did not rejoice in their portion. The latter's Chachmah could not veer people from their way [of always desiring more].
What is the significance of "l'Chol Asher Hayah Lifneihem"?
Rav Sadya Gaon: Those before him 1 .
Rashi: There was no limit to all the good that was in front of them, and all was lost - "uv'Rega She'ol Yechatu" (Iyov 21:13).
Rashbam: The previous generation, subjects of the old Kesil king.
Ri Kara: Previous generations were displeased with a foolish king, and deposed him and made another king in place of him.
Rid: All the generations before these two generations. They had no advantage in this world. It discusses two generations, for the world always needs two generations - the fathers and the children.
Seforno: Also previous generations, most abandoned eternal life and engaged primarily in temporary life. Today, all memory of them was lost.
Metzudas David: Refer to 4:16:1:5.
I.e. those who were subjects of the old king, before the young, poor Chacham became king. Rav Sadya Gaon explains as if it says 'Lefanav' (PF).
Why does it say "Gam ha'Acharonim"?
Also subjects of the young, poor Chacham who became king, who never knew the old king 1 .
Rashi: It is Dor ha'Flagah.
Ibn Ezra #1: Also the previous [old Kesil] king, they were not happy with him.
Ibn Ezra #2: They inherit the current generation, just like the current generation inherited the generation before them.
Rashbam: Also the next generation, subjects of the son of the old Kesil king, do not rejoice in their king, due to his great folly.
Ri Kara: Also later generations will be displeased with a foolish king, and will depose him and appoint another king in place of him.
Rid: Also these two generations [like all generations before them].
Seforno: Also later generations strive [based on the counsel of] the second young Chacham. All his Chachmah is in matters of this world.
Metzudas Tziyon: Also the nation in the days of the second youth's reign will not be satisfied with their good. Even though two Chacham kings ruled over them consecutively, they could not veer their hearts to rejoice in their portions.
It seems to me that so explains Rav Sadya Gaon. (PF)
In what will they not rejoice?
Rashi: In all the good that was given to them.
Ibn Ezra #1: The Chacham who became king 1 . Even though he is a Chacham, the laws of kingship are harsh [on the nation].
Ibn Ezra, Rid: The (this) world.
Rashbam, Ri Kara: Their king.
Metzudas Tziyon: The good that they have.
It seems to me that so explains Rav Sadya Gaon. (PF)
What do we learn from "Ki Gam Zeh Hevel u'Rayon Ru'ach"?
Rashi: This is the end of Hevel and Rayon Ru'ach, when man engages in Hevel, after [the counsel of] his Yetzer.
Rashbam: It is folly that the kings are foolish, and the generations are subjugated to them.
Ri Kara: It is folly, that one loses great good (kingship) via his folly.
Rid: Also now this world is Hevel, like it was initially.
Seforno: Also what is attained via the counsel of the second young Chacham, it is not really important; many are satisfied with these imagined acquisitions.
Metzudas Tziyon: Also the rule of wise kings is Hevel - a broken desire, for they cannot straighten the nation's Da'as to the proper path. There is no benefit, and no fulfillment of desire.