Even efficiency improvements only help up to a point. Just because making things more efficient increased propserity at one point, doesn't mean they will continue to do so. In fact, the very crossing of that point of diminishing returns is what fuels the late-stage capitalism you refer to.

Honestly, I would say that the causality is the reverse.

Late-stage capitalism is enabled by the 4+ decades of the very wealthy taking all the gains from increased efficiency, which gives them the power to turn it into a feedback loop.

Our current situation is not an inevitable and natural outgrowth of the improved productivity of the late 20th century: it is specifically caused by policy changes (starting) under Reagan that allowed for more consolidation, less care for the common good, and more focus on personal self-aggrandizement.

Well, you definitely have a point there. The causality is less clear than I implied for sure with regard to the connection between late-stage capitalism and efficiency.