Is reading a discussion on HN morally defensible? You could be spending that time helping humanity somehow.
Is working at a tech start-up making mobile apps morally defensible? You could be working at a soup kitchen instead.
Is playing a video game morally defensible? You could be assisting volunteering at a shelter instead.
Is having heat in your house morally defensible? You could be turning it off to save money which you could send to help alleviate poverty, though this will require you to endure freezing temperatures in your home.
But it's marketed as somehow important to the survival of humanity, and that's a lie. There are no scenarios, even vaguely theoretical ones, where Mars is more hospitable than Earth any time in the next million years.
I feel almost exactly the same way about Burning Man. To the extent that it's marketed as a dance party in the desert for the wealthy, I think it's a wonderful thing. I respect that it holds important religious services for many people.
When people start talking about how it's an evolution of society, or somehow a solution to the world's problems, I take issue.
FWIW, I would answer the titular question in the affirmative: Yes, a Mars mission is morally defensible. It's just not good for society right now. Still, we should probably do it sooner than later, since we're about to burn our oil reserves. This might be our last chance to get out to the asteroids and build the factories to seed the space economy. But again, from Earth's perspective this is all just entertainment.
Why should we worry about fixing up the problem with lead in drinking water in Flint as long as there's people there living in poverty and starving? We should just let everyone drink leaded water until the poverty problem is fixed.
Why are we wasting money on developing more efficient farming techniques as long as there's people starving? That money should be spend on feeding people instead.
---
This is basically what the article is arguing: that we shouldn't develop new technologies as long as there's people living in poverty. Never mind that the new technologies developed improve the economy greatly and help life more people out of poverty.
Come on. This is bullshit. Musk is also pushing environmentalism here on Earth with SolarCity and Tesla. Going to Mars and taking care of Earth are in no way mutually exclusive.
What is not moral is the fact that it is becoming a sort of big brother show based on no scientific assumptions.
If US put the same kind of money into NASA they did during the Apollo days (so 4% of federal budged instead of 0.5% for a decade or two without changing the plan/goal of NASA with every administration change) a lot of stuff would be possible.
If you had an "infinite" budged you could just build a VERY big space craft in orbit and use that. But the money isn't there so it isn't happening until SpaceX or whoever lowers the launch prices further (SpaceX Falcon Heavy and/or ITS/MCT for example)