Robin Hanson expects that our descendants (whether biological or machine) will lead a subsistence life, that is the resources available to the median individual will not be significantly greater than what is necessary to allow survival. While I tend to generally agree with his contention, I do have a few quibbles.
First of all, the details of what constitutes "subsistence level" crucially depend on the time horizon one looks at. In the very short term (the time needed to write "In the very short term") all I need to survive is the absence of major alterations in temperature, gravity, radiation levels, etc. To finish this post I also need air. To see the Sun turn into a white dwarf I would need to also conquer aging, keep a gigaerg source of energy, as well as end war and petty violence, just to name a few of the likely long-term subsistence-level resources and conditions.
In this context, the word "subsistence" should not evoke images of swarthy farmers plowing their tiny plots behind burros. Galaxy-long linear accelerators might turn out to be indispensable tools for the would-be trillion-year survivor. One might need to understand and explore the string theory landscape to escape the heat death of the universe, or the Big Rip, or whatever doom awaits this little pocket of dusty vacuum we live in.
But, of course, the notions of long-term vs. short-term are themselves dependent on the entity entertaining them. For an E.coli bacterium in exponential growth phase, long-term may mean beyond two to three minutes, since most of the feedback loops operating within this survivor are calibrated in seconds. One must also remember that there is a hierarchy of (potentially) replicating entities, starting with genes, through cells, to multicellular organisms, to superorganisms, and evolution, depending on the conditions, may act on any of those levels (arguments against simplistic group selection theory acknowledged and irrelevant). The time span of subsistence will then depend on the organizational level of the entity undergoing evolution. The configuration space of likely entities into which humans could evolve is huge and varied, and we know too pitifully little to predict the exact trajectory of post-human evolution. This includes predicting the longevity of the dominant mind species in the post-human ecology.
Our minds and bodies have been blindly shaped by evolution but this process couldn't keep up with technological change, resulting in a mismatch between innate desires and actual fitness-relevant possibilities. I agree with Robin that this situation is unlikely to persist over any evolutionarily long time. I expect that soon we will be supplanted by post-human beings specifically designed for survival in a technological ecosystem, and the natural, blind evolution will thus largely end (who would design these entities is an issue I may consider in another post). But, although we can surmise the post-humans will be good fitness-maximizers, we can say very little about their organizational level, physical form, longevity and the actual physical meaning of "subsistence resources".
Thus, even though Robin is most likely right that our descendants will lead a subsistence life, their desires fully aligned with the goal of survival and none of their resources expended on irrelevant frippery, we should realize that their notion of subsistence may be quite sumptuous by our standards. Maybe they will need to expend stupendous amounts of energy just to prove fitness and to discourage predation. Maybe they will explore the most esoteric realms of post-human mathematics to reliably navigate away from cosmological disasters. Maybe their social graces, needed to stay afloat in a forever-changing political world, would surpass the most Machiavellian of our ruling classes. One is reminded of the adaptive immune system, the energetically expensive and extremely dangerous (see autoimmunity) assemblage of B and T cells that constantly churn and evolve, ever on the lookout for new infectious challenges, with no end in sight - running very quickly just to stand in place.
I think there is a good chance that some of the seemingly non-essential attributes of our lives may in the long term become subsistence necessities, at least in some of the newly created ecological niches. Freedom, unbounded curiosity, never-ending exploration, mastery over matter, and owning very large private spaceships with wormhole generators could in some scenarios be the analogue of having a burro and five acres of land - but hopefully much more fun. Time will tell.