Alway, Heb. all days, until the coming of the Messiah, as it is elsewhere limited and explained; for so long, and not longer, this succession might seem necessary for the making good of God's promise and covenant made with David. But when the Messiah was once come, there was no more need of any succession, and the sceptre might and did without any inconvenience depart from Judah, and from all the succeeding branches of David's family, because the Messiah was to hold the kingdom for ever in his own person, though not in so gross a way as the carnal Jews imagined, but in a spiritual manner. A light, i.e. a son and successor. Of this phrase, See Poole on "1 Kings 11:36".
Related Commentaries of 2 Kings 8:19
2 Kings 8:19
19 Yet the LORD would not destroy Judah for David his servant's sake, as he promised him to give him alway a light,c and to his children.