Frankly, I'm wondering if the whole idea of exponential growth is just short cultural time horizons applied to the implications of fossil fuels for energy production, which touched off the Industrial Revolution. The Hubbert Peak holds, although coming out the other side of it resembles a gradual stepping-downward with its own local spikes and valleys (much as there are spikes and valleys in growth and use now, despite a steady upward trend). Fossil fuels still supply over three quarters of the world's energy demand; there hasn't been a nuclear renaissance so far and as much as someone always wants to boost pebble bed, travelling-wave or thorium reactors, innovation and growth for nuclear both seem quite limited on the balance. That might not seem like a big deal now (surely it could happen, right?) but what if that situation does not change appreciably, and world civilization starts transiting down the other side of of the curve, taking a few centuries to do it? What if we never do figure out FAI, or MNT, or fusion, or whatnot? What if that's because the noise of society, geopolitics and history-in-general just don't allow for them to come to pass?

What if the the answer to Fermi's paradox is simply "You'd have to mistake the infrastructural equivalent of a blood sugar rush for an inexorable trend in technological development to even wonder why nobody's zipping around in relativistic spacecraft or building Dyson spheres?" What if the problem is just short time horizons and poor understanding of context?