> ...but is it really the best use of resources to throw $1 billion at reinforcement learning without any similar investments into addressing... <important things>

The precise same sentiment could be made about his plans to go to Mars- people are starving on Earth. But a Martian "backup civilization" might someday save humanity. Similarly, godlike AGI might be mere decades away and apocalyptically dangerous- predicting scientific advancements is generally impossible, but some people working in the field don't think it's an unreasonable possibility [1]. That humanity, via Musk, has put a billion dollars into worrying about this somewhat plausible existential threat seems reasonable, to me.

Musk is not a staid, Bill Gates-style philanthropist optimizing for dollar-for-dollar benefits. He's picked a set of flashy, long-term goals and started manic efforts to promote them. Perhaps it's a character flaw. But given that, I think he's at least chosen well.

[1] http://aiimpacts.org/2016-expert-survey-on-progress-in-ai/