We have covered that, and explained to you, but you are ignoring the answers that already have been given.
You are talking about “what-if” possibilities in the future, but biological sciences deal with the current available evidence & data to determine probabilities, and the evidence are clear, neither the reptile groups & bird groups, nor the mammals group, would become or evolve back to being amphibians or fishes again.
Evolution “evolve”, branching outward and forward, not going backwards.
It isn’t just about being having four limbs (Tetrapoda), of being able to support their body weight, to move more freely on dry lands, and it isn’t just about having lungs that do the works, to breathe.
It is more than that. There are another very important element that distinguish mammals, reptiles and birds from fishes and amphibians.
In their reproduction cycle, the fish groups and amphibian groups have continued to lay their eggs, UNDERWATER, in the AQUATIC ENVIRONMENTS, like the seas, lakes, rivers, etc. Hence, the biologists classified the fishes and amphibians as ANAMNIOTA. All anamniotes - fishes and amphibians - lay their eggs underwater.
The earliest ancestors of the mammals as well as all the current species, and the ancestors of the reptiles and birds, as well as all the current species, are classified respectively as SYNAPSIDA and SAUROPSIDA.
Both Synapsida & Sauropsida are the two clades of AMNIOTA. And Amniota itself is the clade of Tetrapoda that are all animals that are not anamniotes like extant or extinct amphibians.
All amniotes, reproduce in one of two ways:
- They either lay their fertilised eggs on dry lands, as all reptiles and birds do, as well as few species of mammals, the monotreme (Monotremata, an order of Mammalia), like the platypuses and echidnas.
- Or, the embryos and foetuses grow in the wombs, prior to live births, as all mammals, except the monotreme creatures (echidnas & platypuses).
That’s the other thing that separated mammals from fishes and amphibians.
And speaking of amphibians. While amphibians can live in both on land and under water, whenever they need to lay their eggs, they always return to the water, laying their eggs underwater. The eggs from amphibians (as well as fish eggs) lack the amnion - the membrane that protect the developing embryos, hence all anamniotes have to lay their eggs in aquatic environments.
Even more important, when the eggs of amphibians, hatched, the tadpoles (the larval stages of young amphibians) have gills, but as they mature, frogs and toads tends to lose their gills, they breathe underwater, through their skins. The adult salamanders, on the other hand, depending on their species, would either keep their gills, while others lose their gills and breathe through their skin when underwater…as I said, it depends on their species species of salamander.
For the amniotes (reptiles, birds & mammals). The amnion is essential membrane that protect the eggs during the embryonic stage, so that the embryos would grow, when they are lay on dry land, or continue to grow in the female wombs.
When crocodiles, sea turtles or sea snakes, reproduced, they would return dry land, to lay their eggs. I know that turtles buried their eggs, that not only keep eggs warm, but afford the eggs some protection from predators.
Like or not, crocodiles have existed, pretty much as the earliest dinosaurs, and they haven’t evolved much, still living most of the time in water, and yet they would always lay their eggs on dry land, not underwater like the amphibians do. Despite the tens of millions of years, their lifestyle haven’t changed, as they still don’t gills.
Penguins also spend much times in the seas, but when the time, they too would deposit their eggs on either dry lands or on ice, and then sit on them until the eggs hatch. Penguins have grow gills.
Whales, dolphins and porpoises, I have already mentioned in the past replies, always have the embryos and foetuses grow in their wombs for some periods of time, prior to live births, just like most mammals do. None of these cetaceans have needed gills.
It is highly likely that aquatic & semi-aquatic mammals, reptiles & birds would continue their evolution without needing gills.
You keep talking about what-if scenarios, but you are ignoring the evidence and data.
