I've long been convinced that intelligent species exist, but only for very short periods of time cosmologically speaking. I now believe that those species probably exist near the peak of their technological capabilities for only an extremely short time, probably less than 500 years, depending mainly on their rate of reproduction (less fertile species exist longer).

The reason is twofold: first, we develop on spheres where stored resources are axiomatically finite, and secondly our techological development allows us to rapdly dominate all the resources of that sphere. This leads automatically to a situation of overshoot, where the easily available stored resources support exponential population growth, followed by a Malthusian collapse. The time required to enter that overshoot depends on three things - the availability of resources, the overall fertility of the species, and their ingenuity. A reduction an any of these factors leads to a broader curve (slower ascent and descent) without changing its fundamental shape.

I no longer see expansion into space as a saviour. the reason is that the growing ability to exploit local resources causes the population to start exploding well in advance of the development of pinnacle technologies like mass space flight. I also expect that the utilization of renewables would not play a major role in saving a species, because by the time the need to collect such diffuse energy was obvious it would be too late due to exponential population pressure and the depletion of the foundational stored energy sources.

I'm obviously guilty of massive anthropocentrism here, because my primary assumption is that intelligent species arise in conditions much like those we have here. Still, it seems like a reasonable first approximation.