You can take this too far: "Why are we debating housing policies when we aren't in a position to implement them? Why don't we only discuss options available to us, like donating to PACs, writing our representatives, or canvassing in person?" You do still need to figure out whether some alternate world would be worth working for, which is one component in what actions you should take yourself. But its not enough to convince me that global warming is an extremely serious threat to humanity without making the companion arguments for what the world should be doing to stop it and what I should be doing to make that happen.

(Because social programs tend not to work, the main hurdle isn't establishing that a program's goal is worthy but instead that it's moving us toward its goal.)