כִּסְאֲךָ֣ אֱ֭לֹהִים עוֹלָ֣ם וָעֶ֑ד שֵׁ֥בֶט מִ֝ישֹׁ֗ר שֵׁ֣בֶט מַלְכוּתֶֽךָ׃
Ton trône, [fondé par] Dieu, durera à jamais; le sceptre de ta royauté est un sceptre de droiture.
Derekh Hashem
From that which is very discernible about His providence, may He be blessed, is that the foundations of all of the arrangements of His providence are righteous law and precise justice. And [this] is like the matter that is stated (Psalms 45:7), "your royal scepter is a scepter of righteousness." And it is [likewise] written (Proverbs 29:4), "By justice a king sustains the land." However we know that the true desire of the Holy One, blessed be He, is only to do good. And note that He loves His creatures like a father loves his son. Yet by reason of the love itself, it is appropriate for a father to discipline his son in order to benefit him in the end - like the matter that is stated (Deuteronomy 8:5), "as like when a man disciplines his son, the Lord your God will discipline you." So it comes out that the judgement and the verdict itself flows from a place of love; such that the discipline of the Holy One, blessed be He, is not the blow of an enemy or one taking revenge, but rather the discipline of a father that wants the good for his son, as we wrote. Indeed, there are two matters that come out of this: The first is that the discipline itself is sweetened and not harsh or cruel. For the love that is with it mixes the judgement with mercy. And the second is that sometimes when the time requires it, the Master, blessed be He, will completely pass over the letter of the law, and act with mercy. And [this is] like the matter that is stated (Exodus 33:19), "and I will have grace upon whom I will have grace, and I will have mercy upon whom I will have mercy." But behold, since the Holy One, blessed be He, wanted the free choice of man in his actions; and, according to the righteousness of His law, the reward to be paid to a man according to his deeds - He surely, as it were, subjugates His direction to the deeds of man, such that He only cause good or bad according to [man's] deeds. However, in truth, the Master, blessed be He, is surely not subjugated to any law and does not require anyone else and is not acted upon by anything. Hence when He wants to use His preeminence, he surely acts and directs [the world] as He wants, without any compunction or hindrance at all. However regarding [His] normative direction, He directs [the world] according to the subjugation that we mentioned. Yet when His Wisdom decrees that it is appropriate to go beyond the letter of the law, He certainly uses His preeminence to pass over sin and rectify any damage with His great power. It comes out that there are two types of providence here: The providence of God, may He be blessed, and the providence of Dominion and Unity. For behold, He normally presides with normative providence, to judge all of his creation; whereas He [only occasionally] presides with the providence of Dominion to preserve the creation with His power and ability - so that it not be destroyed by the evil acts of people.
Derashot HaRan
And our sages have yet another view in the Midrashim, namely, that this assurance materializes only after the coming of the redeemer, the assurance being that even though the kingdom may cease from his tribe for a certain period, after Messiah comes it will never cease from his seed. "Until Shiloh shall come" is understood, in this context, as "when Shiloh shall come." This is mentioned in Genesis Rabbah (99:10) and in Yelamdeinu, viz.: "'The staff shall not depart from Judah' — this is the throne of kingdom, as it is written (Psalms 45:7): 'Your throne, O G-d, is for ever and ever.' 'And the sceptre from between his feet' — when there comes the one of whom it is written (Isaiah 28:3): 'The crown of pride of the drunkards of Ephraim shall be trodden under feet.' 'Until Shiloh shall come' — when there comes the one to whom the kingdom belongs. 'And his is the blunting of the peoples' — when there comes the one who blunts the teeth of all the nations of the world, as it is written (Micah 7:16): 'They shall lay their hands on their mouths, their ears shall be deaf.'" It is clear from their words that the assurance does not take effect until after the coming of the redeemer. And they expounded similarly (Genesis Rabbah 97): "'The staff shall not depart from Judah' — this is Messiah, the son of David, who is destined to subjugate the kingdoms, as it is written (Psalms 2:9): 'You shall break them with a rod of iron.' 'And the sceptre from between his feet' — these are the inhabitants of Yavetz. 'Until Shiloh shall come' — all the peoples of the world are destined to bring a gift to Messiah, the son of David, as it is written (Isaiah 18:7): 'In that time a present shall be brought to the L-rd of hosts.'"