Shrug, I don't know the specifics of Namor's sterility, but apparently he's not, because he had a child with a non-royal Atlantean, she was killed, and Namor had their child sent away and raised in secret for his own safety, but he was raised to hate Namor.
I think the whole point of destroying Atlantis was just to cut all the strings attached to Namor that had been keeping him in the same place for decades, and allow writers to do whatever they wanted with him. Like having him betray Osborn and join the X-Men, without Namor having to worry about Atlantis facing destruction for his actions.
Also, the Atlantean terrorist cell still existed, Namor only had his son killed, but the Sentry killed them off in Dark Reign, so that removed a lot of the Atlanteans that blatantly hated surface worlders. Because what was left was the members of the Atlantean army that stayed loyal to Namor during the attempted coup and recieved refuge in Latveria, and Atlantean citizens who were suppose to go live with humanity.
Also, I didn't really touch upon it enough, but Kamar was trying to provoke a war and was purposely leaving clues to point the blame at Atlanteans. So Iron Man's army heading to Atlantis was also a reason why Namor destroyed it, and left no trace of any Atlanteans, except Kamar's skeleton.