All the policies that worked in the Boost Phase no longer work.
Natural and human systems tend to go through stages of expansion, stagnation and decline that follow what's known as the S-Curve. The dynamic isn't difficult to understand: an unfilled ecological niche is suddenly open due to a new adaptation; a bacteria evolves to exploit a new host, etc. Expansion is rapid until the niche is fully occupied, and then growth matures and stagnates; the low-hanging fruit has all been picked, and it's much more costly to reach what little is left.
Human economies