What if we have the trajectory of technological progress incorrect? Say, if instead of the rate of technological advance being exponential its actually sigmoidal (or something similar), which I think is possible. For example, in many chemical reactions the rate of reaction accelerates as the reaction happens due to more free energy. But then the acceleration slows due to the lack of available reactants. I think a similar thing could be afoot with technological advance, given that faster machines require more power, which will eventually deplete the available resources, or perhaps create so much waste heat, as to slow down the machines themselves. The most powerful processors today have a heat flux equitable to a nuclear reactor. A fast enough processor will melt itself. Maybe these machines will figure out how to minimize energy input and maximize energy extraction (from the Sun, for example) and cooling efficiency, but still, I don't think its a given that technological advance can accelerate forever. Nothing is infinite, nor can be. Maybe we're on the upslope of a sigmoidal curve and are extrapolating too far into the future based on a small sample size.