Ecological degradation is already preventing many living things (including a lot of humans) from achieving their full potential for self-realization (or their three-score and ten if you prefer).

It doesn't have to go much further for a decline in human population to become evident (as it already is for many other species). If death rate exceeds birth rate by 1% then a population halves in about 70 years (or three human generations).

How you intervene in that process is an interesting question - one answer is not to bother, just let it happen. Another answer is to try and find some parameter to control in order to achieve a more desirable outcome than the default - which begs the question of desirable for whom?

Perhaps from an ecological point of view maximising human population decline in over-consuming territories might be desirable - in which case you could make a strong ecological case for preventing all immigration into developed countries and encouraging developed world emigration to the third world on the grounds that a third world immigrant to the UK will increase her resource consumption to unsustainable levels, whereas a UK emigrant to (say) Mali will almost certainly reduce her net consumption.

Or perhaps not?