Your observation about diminishing returns is an excellent one, but your comparison about space exploration and movie making doesn't make much sense.
The reason these people don't argue that we shouldn't stop making movies until we end world hunger is that (well, most of the time), the making of movies is not funded by the government, but by the private market.
It's a one thing to to argue that we shouldn't do Y at all as a species until we accomplish X ("we shouldn't make movies when people are starving"). It's very different to argue that maybe the government should completely prioritize X over Y for the purposes of allocating tax money ("let's not spend money on space until we can feed everyone").
It should be remembered that movie productions are subject to an impressive array of subsidies and movie companies the beneficiary of a great deal of government policy and enforcement.
That aside, there will always be a better cause to which all resources should be devoted. And usually problems which would result from a solution. Would a more rapidly growing world in which everyone has food but in which nobody has a job be a better one?
The reason these people don't argue that we shouldn't stop making movies until we end world hunger is that (well, most of the time), the making of movies is not funded by the government, but by the private market.
It's a one thing to to argue that we shouldn't do Y at all as a species until we accomplish X ("we shouldn't make movies when people are starving"). It's very different to argue that maybe the government should completely prioritize X over Y for the purposes of allocating tax money ("let's not spend money on space until we can feed everyone").