Hello Denis
Life is an existence proof. If life can do something, we can hope that technology can do it too. Life has existed on Earth for a long time. Your ideas are one way that technological civilization might do so also. However, there are other possibilities that score better on a utilitarian scale.
For example, Lewis [1] estimates that there is enough material in the asteroid belt to build habitats for 10,000,000,000,000,000 people. (10,000 x 1 trillion) - - probably an overestimate. If we assume that every human life has its share of good, that is a lot of utility. Meltzer et al [2] shows a possible method for construction of these habitats. Armstrong and Sandberg [3] show a possible method for settling, not only the asteroid belt or even the galaxy, but thousands of galaxies. Anderberg's essay in this contest [4] suggests a method that might result in even higher utility.
These all are forms of singularity and require exponential growth. Exponential growth sometimes hits limits. Nevertheless, even if we assign these a fairly low probability, they still have a humongous expected value (probability times value). That suggests that they are worth at least some attempt to make them happen. I suggest that even a future requiring drastic cutbacks should have some hope and some R&D to support that hope.
[1] John Lewis, Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets, Perseus Publishing, 1997, pg. 194.
[2] Philip Metzer et al, "Affordable, Rapid Bootstrapping of Space Industry and Solar System Civilization," Journal of Aerospace Engineering, April 2012.
[3] Stuart Armstrong & Anders Sandberg, "Eternity in six hours: Intergalactic spreading of intelligent life and sharpening the Fermi paradox," Acta Astronautica, Aug-Sept 2013.
[4] Tommy Anderberg, A Future Brighter than 100 Trillion Suns, FQXi essay contest.