No problem was ever solved by deliberately reducing the potential number of people available to solve it. If humanity faces resource constraints, the way to solve them is to get as many net-positive-resource people as possible working on solutions.
One person could devote one lifetime to solving a problem. Or, they could have 4 kids, everyone devotes half of one life time (the other half to raising kids), and now there are 2.5+ lifetimes of effort spent on the problem.
We aren't facing resource constraints, we're facing overuse of resources. The problem of overpopulation is not one of an inability to feed, house and water people - the problem is the impact we have upon our own environment.
The single greatest thing we could do to improve our species' odds of survival today is reduce ourselves in number. Because we are currently causing a mass extinction event.
One person could devote one lifetime to solving a problem. Or, they could have 4 kids, everyone devotes half of one life time (the other half to raising kids), and now there are 2.5+ lifetimes of effort spent on the problem.