Psalms 70-72 were deliberately placed at the end of the second Davidic collection where they are intended to be viewed as prayers of David from the time when he was becoming old and about to yield the throne to his son Solomon. Psalms 70-71 are in fact a single unit, composed to serve as a conclusion to the collection. If this hypothesis is extended to include Psalm 72 as well, it may help explain why this text presupposes the continued existence of the monarchy, even though it actually dates from the postexilic period.
Auwers, J.-M. (1994). Les Psaumes 70-72. Essai de lecture canonique. Revue Biblique, 101(2), 242-257. https://hdl.handle.net/2078.5/154992 (Original work published 1994)