Really x-risk mitigation is for dessert, though. There are so many failures of our society that are lower-hanging in terms of how rational you have to be to see them.

I'd say it is more "so you get to dessert"! There as a backup behind "create an FAI and thereby cure death". (So I'm being a bit reckless and blurring my categories when I put Xrisk at 50%.)

That $10b on life extension sounds about right!