ITER's main principle for future viability is hope. They hope that once a functioning break even plant is operating, they will learn, and others will learn in other fields, and be able to develop something closer to cost effective/economically viable.
This argument is based on pointing a finger to past endeavors where the initial concept build was not economically viable, however follow on science and engineering was able to transform it into something viable. The main counter-point is that Tokamak physics demands massive device scale for break even and net power. And the physics are not anticipated to change... sooo...
The hope I see, is that in some future, where the global economy has expanded beyond earth, the relative cost and scale to build a Tokamak power plant becomes an acceptable fraction of resource expenditure. However, this is predicated that the cost per unit power is comparative or beats other means (not likely in my opinion at this point).
The coming Energy Age (as I like to call it) is what will drive human expansion into and beyond the solar system. Without it, we are not going anywhere except some limited forays into the solar system, and fate implies we start and end here in our system. Think of it as a race. We are trying to beat the solar system's, galaxy's, and universe's attempts to kill us by getting out of the way first. Without real sustainable energy to move us around and support us while doing it, it is not going to happen. Our clock will run out.
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)
What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)