When Krang raised the banner of rebellion and declared he would overthrow Namor's rule—things had not actually reached such an absurd point yet.
Krang was certainly a very powerful Atlantean. In fact, he might have been the strongest Atlantean after Namor, but the gap between him and Namor was enormous. If Namor had understood why his army had suddenly betrayed him, he could have dealt with this guy on the spot.
Besides, Namor did not know why those surface dwellers were so confident. Namor was merely a little hot-tempered; he still had a brain. Once he realized something was wrong with the other side, he swiftly returned to Atlantis's royal city, intending to discuss matters with his loyal subordinates first.
Yes, as the Lord of the Seven Seas, Namor had a group of obscure superpowered subordinates—people like Killer Whale Man Orca, and the genetically modified Tiger Shark who had undergone Namor's genetic alterations. This group, known as the "Abyssal Defenders," could be considered the "Atlantean version of the Avengers," and they were Namor's most loyal followers.
In any case, they were far more loyal than that traitor Krang.
Namor swiftly gathered his subordinates and convened a meeting. He had the Witch in the group handle the magical issues, while Atlantis's scientists investigated the weapons used by the surface dwellers.
He did not believe Krang's claim that "Namor is too weak toward the surface dwellers." Namor might indeed have been unwilling to formally wage war against the surface for certain reasons. But he would never believe Krang's nonsense. If he had been serious, the first thing he should have done was turn around and slaughter those surface dwellers.
"And now, the surface dwellers' oil mines are proceeding with construction in an orderly fashion, which is enough to prove that Krang merely