I agree with what I take to be the main point of your argument, which is that OpenAI will use the money for a variety of different things, many of which aren't averting the apocalypse, and that those things are good. I think it's important to note, though, that Andrew Ng's perspective is far from a consensus one. For one thing, overpopulation on Mars is something that can easily be dealt with when it happens, whereas an intelligence explosion is much more of a flipped switch. For another, though there is a
lot of uncertainty in the expert forecasts, a non-negligible amount of experts think there's a reasonable chance of human-level AI within the next 10-20 years, and that's absolutely something we should be worrying about right now. In a 2014 survey of the top 100 most cited living AI scientists[1], the median estimate of the year at which there was a 10% chance we'd have human-level AI was 2024. Multiply that by whatever chance you think human-level AI leads to an intelligence explosion and potential apocalypse, and then multiply it by the impact of that event… I think for most reasonable inputs you get a pretty huge EV as the result.
[1]: http://sophia.de/pdf/2014_PT-AI_polls.pdf