You also need to factor in the history of life. Basic body plans evolved very early on in life, and further adaptations have been built on top of these early adaptations. This makes it difficult to evolve a new basic body plan since so many other adaptations now depend on what evolved before it.
This is another one those things that evolutionists like to say, without ever being called on it, or thinking about the reasonableness of this presumption. As if evolution started with a simple organism, then kept building upon it, and yet no new simple organisms started again, and again?
How come 5000 years ago another simple celled creature didn't start evolving to become more complex, with another entirely different foundation. And how about 100 years ago? Or 25 years ago? Was is your theory only something that has happened in the past.
Its another black mark against the many your theory can't overcome.