Stallman might not have thought this through or not realized how the pressues of evolution will eventually nullify his choice. For all those who make a conscious decision to avoid having kids, those who are genetically fitter at resisting the urge to reproduce are therefore less likely to pass on their genes. Also, of course those who don't bother resisting or have less genetic inclination to avoid reproducing are more likely to. Thus the next generation will inherit the genes of parents with less resistance to reproduction. Thus if the theory of evolution is true, and all other things are equal, the next generation will be more likely to reproduce. This cycle would repeat until the genes of those who decline to reproduce to save humanity are no longer in the gene pool. Thus Stallmans choice is only effective over a very short number of generations.
Not only that, if those who are more likely to decline to have kids is a good quality, they are actually hurting future generations by removing themselves from contributing it to future generations. This is of course their right.
This is not a new idea, the idea that population controls are doomed to failure is main idea of the novel 'The Mote in God's Eye' by Niven and Pournelle.
The problem is real of course. However, when considering reproduction, which is so intimately tied with evolution, perhaps one should be aware of the consequences of evolution or possibly even use it when trying to solve the problem. Unless those who have fewer offspring are more likely to pass their genes on to the next generation, those in the next generation will have genes that favor more offspring.