"Shall never lack" is pretty plain.
It is plain that it is not speaking of any temporary gaps, otherwise the prophesy would have failed a long long time ago, as there were gaps of waiting between some of the kings that ruled both Judah and Israel in the OT.
Not to mention that there was a time that Israel was ruled by judges and no kings at all until GOD would provide them a king of His choosing (which would be David of the tribe of Judah).
Israel complained that they didn't want to wait for a king.
They wanted one now.
GOD warned that it would not be wise not to wait by choosing one now.
They didn't heed the warning to wait for GOD's choice, and they got Saul (of the tribe of Benjamin, by the way).
Things seemed great at first, but later didn't go so well for them, and they were losing battles.
Many of Israel were abandoning Saul and his rule and Saul just couldn't keep them united.
Not only did Saul have to battle enemies from without Israel, but was also battling with enemies from within.
Finally, GOD's real choice for their king was enthroned - David of the tribe of Judah - the one they should have waited for.
Under David's rule Israel won battles from enemies from without and were all united in a kingdom finally at peace.
It was David of the tribe of Judah's rule that was promised to be forever, not the rule of Saul of the tribe of Benjamin.
And as we know, in the end times, Israel will make a pact with a ruler of the people's choosing - the AC.
Seems fine at first, they get a temple and can once again have their sacrifices in Jerusalem, etc.
But then things go horribly wrong and they are warned to get out and flee!!!! (Scattered again.)
And then the King of GOD's choosing shows up - the promised one descendant from the very loins of King David - the Lion of the tribe of Judah, King of Kings and Lord of Lords.
Winning over enemies and uniting all of Israel and living in peace.
Not a word about that happening in some far off Island.