Ignoring the fact that the number of people working on X may be in decline, and assuming that X is a scientific discipline, however parochial, there are some things to consider:
1) Much, or at least some, of the knowledge a person has will be in their publications. This knowledge will (should) not be lost and is available to the scientific community in perpetuity.
2) Skills such as manual or mental dexterity of any kind require practise, and provided some documentation exists of how to perform the task, repeated practise should do the trick. Perhaps it's dissecting something, perhaps its identifying something using a key.
3) What's left? Unpublished/unpublishable knowledge!
It could be argued that this knowledge should be, or should have been, published but that's not really a satisfactory answer. There are pieces of knowledge that in isolation are useful to know, but would not be publishable alone. This knowledge is outside of the concept of a publon, it's a metaphorical quark.
Some platforms do exist for getting these ideas down in digital ink - one of the ideas we used to throw around in the early days of Scratchpads was creating a tool for taxonomists to share everything they might know and want to share (a brain dump).
It is impossible to predict what will be useful to who, some of the most read posts on this blog are quite arcane in their own way (Processing and USB ports /dev/ttyACM0, /dev/ttyACM1, .... doesn't sound riveting, even to me). The fact is they may be useful to someone - and they don't take long to put online (the blog post I just mentioned was actually as much an aide memoir to myself as an intentional sharing of content).
Some may argue that the real skill of an expert is in figuring things out for oneself, and indeed this is a crucial part of becoming an expert in anything. I would argue however that this exploration is potentially more fertile if it is done in the process of creating new knowledge, not just the rediscovery of what people used to know.
It's a pleasant surprise then to see that a 104 gardener is sharing his knowledge, learnt over nearly ten decades, to whoever might ask for it: Gardener, 104, takes to Twitter to share horticultural tips. 140 characters is sub-publon in size - but I'm betting the information will still be useful to many.