Ok, this is getting into macro scale now. They use entirely different filesystems, boot sequences, package databases, and low-level C libraries. So, in that car analogy what you are describing would effectively be changing out 1 or a few parts at a time and iterating over that until no original part exists and then saying you turned a car into a truck because you kept the car radio the same.
Maybe this is a matter of perspective, but at that point I would say you didn't turn a car into a truck, you just built a truck.
EDIT: I would say most people's constraint on being able / not able to convert distros is "within reason". Without that, we might as well just talk about kernel versions because that's ultimately what Linux is not the DEs and package managers and etc,etc.
To your original point that you can't turn one distro into another (which you seem to now disagree with), you can but not always "within reason".