On a TV documentary on the future plans of all the space agencies for a manned mission to Mars, with the date aimed at being sometime from 2118, the ISS was stated as being an extremely factor in the planning by NASA.
The ISS's other function, as planned by NASA, is to be Earth's first orbital dry dock, as the proposed Mars Mission Craft will need to be assembled in space, due to its sheer size. It was pointed out that all the possible designs looked at so far, are incapable of being launched from the Earth's surface..... how exactly does one find a launch gantry able to cradle a space craft between 1/4 miles and 1/2 mile long anyhow??
The ISS isn't even near halfway built, so far, as there is more to be designed and added on, mainly construction facilities.