I agree that it's not impossible, but it's definitely very late in the cycle to start thinking about PhD applications, and the claim that it would be more helpful to make the case for a PhD to people earlier in the cycle seems totally reasonable to me
The scope is limited relative to what you seem to want, but I thought I might mention the book Theories of Distributive Justice by John Roemer. It walks through several popular philosophical approaches to distributive justice like utilitarianism, maximin principle, etc at a formal level, and discusses relevant results that have been proven about them. I've only leafed through it though, so I can't necessarily vouch for it, but you might want to check it out.
See also: population decline discourse