Why does it say "Hashem Melech Olam va'Ed"?
Rashi: This will be after "Goyim will cease from His land."
Radak: When You will judge Resha'im who oppress Yisrael in exile, You will be king forever, and no one will rebel against You - "v'Hayah Hashem l'Melech Al Kol ha'Aretz" (Zecharyah 14:9).
Malbim: If the entire country rebelled against a mortal king, he cannot kill all of them, for then he will not be king - there is no king without a nation. Hashem's kingship does not depend on a nation, for he is King of the world. His kingship is eternal; it does not change, even if nations perish from His land; His kingship depends on Himself.
What will happen after Goyim will cease from His land?
Refer to 10:16:1:1.
Radak: The kingship will return to Yisrael.