Well first of all i must point that i don't have much knowledge when it comes to biology ( also my English isn't that good so would be happy if my mistakes were corrected).
After i finished the 11th chapter of the book " the greatest show on earth" i couldn't help but wonder about the reason for which cetaceans didn't develop gills, and the first think that came to my mind is: the functions they acquire from the transformation of gills ( into what they are now, and here i will be discussing larynx obviously since it is what have been introduced to me through this book) are more important than developing gills.
So here is how i looked at it: the functions they have acquired is indispensable for them, unlike what i found when i looked in the internet , where everybody was comparing gills and lungs to see which is more efficient for survival
From what i know ( mostly about dolphins and whales) communication is really important for cetaceans and also they use echolocation so an individual who has lost such an organ as the larynx would be disadvantaged in both reproduction and survival. this of course assuming that it will revert to having gills and not redevelop new ones ( since from what i understand the probability is higher this way).