I think it's important at that point to remember that while our growth in the sciences and tech. does derive FROM our ancestors' excess derived value, that it also frequently contributes back INTO our production and access to energy. We don't need to reinvest the energy that it once took to concieve, devise and testout, prototype a steam engine, waterwheel or a cotton-gin, but those creations and their many offspring will continue to offer us ways to get more work done than we could have in simpler agrarian conditions. Many of these tools still require energy to forge and build.. but our technical developments are nonetheless cumulative, and are recorded.

As long as we have access to those records, then we have access to the 'interest earned' on the surplus energy, long since spent, that created them. We have also added that to the now-prodigious canon of human technological development over the last 30 to 50 centuries, and from which we can find other combinations that will be possible with a good bit less initial energy than if we had to earn those advances all over again from our current store of excess energy.

DISCLAIMER: I know.. 'Technology can't save us', and I don't think that this canon of human technical achievement is any kind of a guarantee of Safety, Happiness or BAU. But I also think that our tools and our tool-making history is effectively an organ of our species at this point, and that it is viral (self-replicating), it regrows when it gets cut off (see; Dark Ages, and then what happened AFTER them..) and seems to be subject to the laws of Survival and Development of the fittest, and so grows, regrows, branches and mutates freely. Like DNA and mRNA, it has widely dispersed copies of itself, AND Blueprints and verbal descriptions of itself that can be read, translated, reverse-engineered and modified by sufficiently aware humans with a pinch of literacy, training or time. (Pick any two).. oh yeah, and enough chutzpah at the end of a hard day just surviving in which to work on this.

Bob