1)
Why does it say "Gam Brisi Sufar Es David"?
1.
Radak (20): Just like you cannot annul the Brisim of the day and night (that they not cease), you cannot annul My Bris with David, that his kingship will endure. The same applies to My Bris with Aharon and his sons.
2.
Malbim (20): The Bris of kingship and Kehunah is essential for the continuity of the world, just like the laws of nature are essential. Just like My Bris with existence cannot be Batel, the Bris of kingship and Kehunah cannot be totally nullified. [Mashi'ach] must come within the time limit for redemption.
