I think you need to distinguish different parts of lifestyle. iPhones for everyone? No problem. Air-conditioning? Hurry up with commercial-scale fusion power. Beef for everyone? No way.
In a recent related thread, Stranger On A Train suggested that present population is already much too high in the long-term. Brief excerpts:
Even given the technology of grain cultivation that gave rise to modern high density civilization, the current human population has unambiguously exceeded the natural carrying capacity of the Earth in terms of food production. … Without the process of artificial nitrogen fixation, reserves of nutritionally available nitrogen (primarily from manure and guano) would not sustain current levels of agriculture. A similar limitation are naturally occurring phosphates.
Another fundamental limitation are natural sources of potable water. … modern agriculture currently uses unsustainable volumes of ‘fossil water’ from underground aquifers well beyond rates of replenishment, and the overuse is actually compacting the aquifers so that they cannot carry the same future capacity…
Of course, one can posit some future technology that could replace both of these in a truly sustainable fashion; for instance, methane production from industrial agriculture and waste treatment could recapture nitrogen for use in the Haber–Bosch process without using non-renewable sources, and water can be filtered and cleaned for reuse by reverse osmotic processes. These would allow essentially infinite growth (subject ultimately to fundamental material resources and space for the population to live) but all of this takes more energy and (especially in the case of water) transportation to the user site, which places additional burdens on a fuel economy currently dominated by non-renewable sources.