Second, the remaining retained knowledge was locked inside the heads of a few people and even that was slowly evaporating over time. Third, there was no process documentation of how that past project was done. Almost everyone on the team was starting from scratch and probably repeating many of the mistakes of the prior project with nothing to tell them otherwise. Had they documented the prior project, their chances of retaining some of that productivity would have increased. Instead they were left with little or no value or productivity from their prior, costly learning experiences.
Lets look at another project where documentation had worked.
Several years ago I worked with an engineering team from the start of a project. Most importantly we kept it in chronological order. As problems came up they were recorded, for example the specification needed changes, the software tools did not work properly, the design needed to be redone, the library database needed improvements, and so forth. Everything was recorded, the problems, the assumptions, the solutions, and any unusual techniques. I was responsible for assembling and reviewing the document throughout the six months the design took to complete.