But not forever - David had been distinctly promised that God should never fail his seed, whatever their shortcomings Psalm 89:28-37. The fulfillment of these promises was seen, partly in the Providence which maintained David's family in a royal position until Zerubbabel, but mainly in the preservation of his seed to the time fixed for the coming of Christ, and in the birth of Christ - the Eternal King - from one of David's descendants.

But not for ever - They shall be in affliction and distress till the Messiah come, who shall sit on the throne of David to order it and establish it in judgment and justice for ever. Jarchi says, on this verse,
"When the Messiah comes, the kingdom shall be restored to the house of David.

And I will (r) for this afflict the seed of David, (s) but not for ever.
(r) For this idolatry that Solomon has committed.
(s) For the whole spiritual kingdom was restored in the Messiah.

And I will for this afflict the seed of David,.... For the idolatry Solomon had been guilty of, and connived at:
but not for ever; for when the ten tribes were carried captive, the kingdom of Judah flourished under Hezekiah, Josiah, &c. and though the tribe of Judah was carried captive, yet it returned after seventy years captivity, and had rulers over it of the seed of David; and especially to the Messiah has God given the throne of his father David, of whose kingdom there will he no end, Luke 1:32, and Jarchi's note on the text is,
"for in the days of the Messiah the kingdom shall return to it,''
the seed of David; and Abarbinel says, of a truth, at the coming of our Messiah, this prophecy will be fulfilled; but the true Messiah is come already, in whom it is fulfilled; see Kimchi and Abendana, who refer to Ezekiel 37:19.

For this - For this cause, which I mentioned 1-Kings 11:33. Not for ever - There shall a time come when the seed of David shall not be molested by the kingdom of Israel, but that kingdom shall be destroyed, and the kings of the house of David shall be uppermost, as it was in the days of Asa, Hezekiah and Judah. And at last the Messiah shall come, who shall unite together the broken sticks of Judah and Joseph, and rule over all the Jews and Gentiles too.