We can say that this is an impossible number, that in no foreseeable future could all the pertinent corporations be annihilated altogether, but this is not the question. Can it happen? Probably not, but there is no universal law which states that the great industries of the United States of America are immortal. So much might have been said of those of the Precolumbian Mesoamerican empires, and though their underlying cultures remain long after the last ruling family fell, nevertheless, they fell.
The bottom is 0. If you don't want to hear the answer, don't ask the question.