> Plenty of land mammals have gone back to aquatic living but they don’t regain gills so they still go back to the surface.
Breathing air is probably a superior system anyway, since mammalian fish utterly dominate gilled fish in nearly every respect. What gilled fish can dive like a sperm whale, acquire the bulk of a blue whale, or the agility of a dolphin?
What’s the point of diving like a sperm whale if you can just stay at the bottom of the ocean? As to overall performance the fastest swimmer is the Indo-Pacific Sailfish clocked at 68 mph.
The blue whale is more bulky and faster than a whale shark, but young whale sharks can simply dive too low to be at significant threat from Okra. So breathing has real disadvantages.
What's the point of having gills to stay at the bottom of the ocean if you're going to get dominated by sperm whales anyway? I don't know if sperm whales eat young whale sharks, but they certainly could if they wanted to. They can dive deeper than any whale shark.
Breathing air is probably a superior system anyway, since mammalian fish utterly dominate gilled fish in nearly every respect. What gilled fish can dive like a sperm whale, acquire the bulk of a blue whale, or the agility of a dolphin?