John Gill Commentary 2 Kings 8:19

"Howbeit Jehovah would not destroy Judah, for David his servant`s sake, as he promised him to give unto him a lamp for his children alway." — 2 Kings 8:19 (ASV)

Yet the Lord would not destroy Judah for David his servant's
sake
Not for his merits, but for the mercy he assured him of:

as he promised him to give to him always a light, [and] to his
children ;
or a kingdom, as the Targum; therefore he would not utterly destroy the tribe, nor suffer the sceptre or government to depart from it till the Messiah came, see (Psalms 132:11Psalms 132:17) .