That's not how evolution works...

Whales and humans evolved gills, it's just the gills evolved into jaws and ears while we were still fish.

We still have "gills" they just no longer perform their original function because lungs did a better job out of water.

When whales went back to the water, they kept ears and jaws, but never got their other gills back. It could have happened tho. Every single embryo gets "gills" that later become jaws and inner ears. But could hypothetically just develop gills like Kevin Costner in Waterworld.

The real weird one is species of crabs that can breathe on land if their gills are wet, but drown if underwater too long because that was the trade off

Well, best of both worlds.
There is a reason maritime mammals feature the biggest animals to have ever lived, and on top of that not only some of the most relaxed species to exist, but also a bunch of the most successful hunters ever.

Meanwhile mudskippers 🫪

There is not much oxygen in water, especially at higher depths. If you can get oxygen directly from the air you have a big advantage.