There's no law that says we have to find The Worst Problem, devote all our resources to fixing it, and totally ignore every other problem that humanity has while The Worst Problem persists. Such a policy would lead to a rather horrifying world.

Something similar has been seriously argued here for donations to charity: you should donate all your money to the single charity that would do the most good (unless you're a millionaire who can donate so much money that the charity will reduce the size of the problem to below the size of another problem).

http://lesswrong.com/lw/elo/a_mathematical_explanation_of_why_charity/ http://lesswrong.com/lw/gtm/when_should_you_give_to_multiple_charities/ http://lesswrong.com/lw/aid/heuristics_and_biases_in_charity/

Some of the comments have good arguments against this, however.