The preamble of the US Constitution is as follows:
"We the people of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America."
The word in the preamble that indicates that the framers wanted the constitution to last forever is the word 'posterity'. Posterity refers to all future generations.