Rashi: He was alone in His grandeur, and also now "va'Yeshev Hashem Melech l'Olam" - alone. The idols will lapse.

Rashi (from Zevachim 116a): When the Torah was given, the nations thought that Hashem will destroy the world, like at the time of the flood. Bil'am (Radak - angels) said no - He already swore not to flood the (entire) world again. Rather, He is giving Torah to His nation.

Radak (11) #1: At the time of the flood, people did not recognize Hashem's kingship. They said "Sur Mimenu u'Mah Yif'al Shadai Lamo" (Iyov 22:17). He brought the flood on them and destroyed the world. He left No'ach and his sons, Tzadikim; those after them knew that Hashem rules over all. Kingship is His; He created the world, and He destroys it when He wants. Also in the days of Mashi'ach, nations that harmed Yisrael and did not fear Kel, He will judge them. Yisrael will remain, and those Nochrim who will repent. They will say "va'Yeshev Hashem Melech l'Olam"!

Radak (11) #2: Destruction of the nations is metaphorically called a flood - "v'Hacharavti Es Yamah" (Yirmeyah 51:36). The nations are serene and full of all good in this world, "ka'Mayim la'Yam Mechasim" (Yeshayah 11:9).

Malbim: Even if Hashem sat at the time of the flood, even if the current deluge and clamor will increase until it will be a flood to destroy the entire world, Hashem will be King forever to judge the entire land. "Hashem Yimloch l'Olam va'Ed" (Shemos 15:18) - also after nations perished from His land.