“ What keeps the software alive are the programmers who have an accurate mental model (theory) of how it is built and works. That mental model can only be learned by having worked on the project while it grew or by working alongside somebody who did, who can help you absorb the theory.”
wiki says:
“ Tacit knowledge can be defined as skills, ideas and experiences that are possessed by people but are not codified and may not necessarily be easily expressed.[5] With tacit knowledge, people are not often aware of the knowledge they possess or how it can be valuable to others. Effective transfer of tacit knowledge generally requires extensive personal contact, regular interaction,[6] and trust. This kind of knowledge can only be revealed through practice in a particular context and transmitted through social networks.[7] To some extent it is "captured" when the knowledge holder joins a network or a community of practice.[6]”
The distinction appears to be the implication of OP that the ‘theory’ is not communicable, thus “first generation” workers need to be around. But you can clarify (am interested in your views) (a) what’s your crit of TD and TK, and (b) the matter that I missed, the “something very important” (which is not TK transfer).
p.s. “we know more than we can tell” is pretty much the tldr of T.D.