I think existential risks to our species of much higher magnitude exist today. Climate change and pollution for instance. I think a "possible existential threat in the future" is a weaker case for philanthropy than existing ones.

Both of those issue cannot be solved by throwing money at them, especially the first. It requires political forces.

Then again, the money could be used to buy politicians' votes in favor of environment-friendly policies. It's so blatant in the U.S. these days.