Increased Productivity - So far, technology has made each person much more productive but it has also made them more expensive on the environment. I don't see that changing. And with more people being able to do more, the base level of required population drops. so a self chosen drop might be optimal for society's continued well being.
Life span increases and uploaded minds - If I were alive in 5000 years I would be suprised if the average life span had not been extended by a significant amount. Not to mention the possibility of minds hosted [2] on computations carried out by photons, particle spin or whatever phlebotinum they have.
Environmental stress - With people living longer and getting ever more expensive for the earth to host maybe it is ok that we are already self correcting now. Food may not be a problem but exponentially growing economic productivity could be. A lot of the environmental damage would be greatly reduced if there were less people[3]. This is a very dangerous line of thinking to traverse though and must be done only by those with a temperment strongly fortified with wisdom. Too rational an approach at scale will not be good for the individual.
Designer Babies - I read that people are already filtering baby genders and against diseases while they are still embryos [4]. It is only a matter of time till filtering graduates to optimization. Ignoring the possibility of a brave new world or gattaca scenario and the subsequent upheaval then destabilization of society as the have nots revolt, such a population could easily optimally repopulate the world at a controlled rate without people having to marry to make babies.
[2] The idea of mind uploading I find thrilling but very uncomfortable. Even if I were one such uploaded being, the difference between that being and I would dwarf the difference between me and a bacterium. I may as well have died.
[3] Assuming it is not the endpath of civilization to completely dismantle its planet for its needs. and then go invisible.
[4] http://singularityhub.com/2009/02/25/designer-babies-like-it...