Somehow overpopulation didn't make his list of existential risks? Seems like a glaring hypocrisy for someone who claims to have an interest in saving the species.
Overpopulation is not an existential risk: it does not kill the entire human race or permanently curtail/destroy potential. Overpopulation sucks and can cause serious problem for those affected, but it does not threaten all of humanity and to some extent solves itself - you can't go beyond the Malthusian equilibrium.
Not only is overpopulation not an existential risk, it's not even really a risk: look up the demographic transition.
I think it's a very serious issue. Clearly the population will balance itself at some point, whether by education/moderation or mass-starvation. However, I believe we have a finite window of opportunity to make a number of critical scientific and social advancements in order to continue our species' progress in the next few centuries and beyond. This window is defined primarily by the availability of materials and energy which are necessary to support technological development at our current scientific frontiers. Increasing population, especially in the developed world, consumes resources faster and thus shortens the span before many critical materials become scarce. Better extraction and synthesis will likely become available in the future, but first we need to get there.
I am not arguing that we shouldn't have children at all, but responsibly limiting our population seems like a critical supporting factor in any qualitative advancement we want to make as a species. This is especially true since much of the world doesn't seem likely to do the same any time soon.
I think you make some good observations, but I'd wager if you think these statements through a while longer, you'll adjust your position. Jaan (a single high fitness human) having multiple children doesn't connect directly to the real issue of overpopulation. The carrying capacity of the earth (with efficient use of vertical space) is orders of magnitude large than our current population. Survival-necessary resources are not scarce. Depletion of scarce resources causes serious economic and societal impact, but can you name one irreplaceable scarce resource? We're almost out of oil but tech will transition to alternatives soon. I worry about byproducts of overpopulation, like multiple antibiotic resistance bacteria and calcified local optima in social systems.
Your comment didn't address any of my points. Your 'finite window of opportunity', besides being completely unjustified and probably wrong on a long-term time-scale, does not relate to overpopulation rather than, say, Peak Oil, and you ignored my point that the risk simply isn't there due to the demographic transition - which makes your claim that "Increasing population, especially in the developed world, consumes resources faster" simply wrong because places like Japan or South Korea (developed countries by any measure) are projecting decreases in population soon, and even the USA is growing mostly due to immigration.
People often wake up to these things at a certain age or certain phase in life. It's good to encourage and welcome people to do what they can, instead of demanding a 100% conversion to captain planet and perfect history of ecological life choices.
Not only is overpopulation not an existential risk, it's not even really a risk: look up the demographic transition.