Of course it's unsustainable. 8 billion people need at least 4 planets worth of ecosystem services to maintain itself without permanent degradation. But even if we manage to reduce the population to 2 billion before the biosphere is degraded, we're still going to face annihilation by the cosmic RNG (GRB's, CME's, long-period comets, supernovae, extrasolar objects, asteroids, supervolcanoes, etc.).