It takes a lot of imagination to think that the founders had any vision of the laws that the commerce clause would be used to justify today.
It took me a long time to realize that whether or not the constitution was designed or intentioned as a "living document" is completely irrelevant. What matters is how the process actually works today, and it works as if the constitution is a living document, therefore it is a living document. See also: Duck typing.
They needn't envision those laws, just as they needn't have envisioned automobiles or the internet. You could think of the Constitution establishes axioms. Just like math, what arises from those axioms can be boundless in complexity, but that has no bearing on the axioms.
It took me a long time to realize that whether or not the constitution was designed or intentioned as a "living document" is completely irrelevant. What matters is how the process actually works today, and it works as if the constitution is a living document, therefore it is a living document. See also: Duck typing.