The clearest such case historically is food production: what share of the population is dedicated to feeding the population, vs. literally everything else? https://ourworldindata.org/employment-in-agriculture shows that this has declined enormously over time, and I'd describe that as the defining feature of modern industrial civilisation.
(though I think that your question is somewhat ill-posed and 'extra value' is almost undefined, it's still pointing to some interesting questions about the structure of the economy)
I don't think this is a useful simplification. The variance in how many people benefit, and how deeply they benefit (or can in the future be expected to benefit) is many-dimensional.
Further, most of your scalable examples end up having an inverse relationship between how many COULD be served and how many are ACTUALLY served, let alone how significantly. Writing is a tough gig, for instance - not only are you competing globally, you're competing with existing works. And very VERY few writers have a significant impact on a reader's life.
sure, I agree that writing is a tough gig and the distribution of what is read how much is pareto, still however the writers contribute to the chance that they improve the top writings that are read the most.
I think I'm much less interested in how deeply poeple benefit, but more in how many of them can potentially benefit and whether this scales roughly with the effort e.g. professions where by spending X effort I can serve Y people and if I wanted to serve 2Y people I would have to spend 2X effort (chef/teacher/hairdresser...) don't fall into the same category as writing.
Maybe a better way of thinking about it is as follows: If additional new 1000 people are added to the population with a usual distribution of skills/professions....what portion of work they do would be contributing to the rest of the already existing population vs just work needed for them to self-sustain and cater to themselves (obviously with substitutions - e.g. if someone from the "new" people cooks food for the "old" ones, but someone from "old" has to cook for someone from the "new" - this does not count as contributing).
Imagine a simplified model of the world, where the population suddenly increases by 1000 individuals. Of these newcomers, perhaps 50 will become policemen to ensure safety, 30 will serve as doctors to maintain health, 25 will take on roles as teachers to educate the children among them, 20 will become hairdressers to uphold appearance, 5 will work in factories to produce additional cars, and 1 might become a mayor, among other roles (these numbers are, of course, entirely arbitrary).
The common characteristics among these professions is that they primarily cater to the needs of the new 1000 people, and thus, their numbers must roughly correspond with the population growth. If the new 1000 individuals were to vanish suddenly, the absence of these workers would likely have minimal impact on the rest of the world.
This contrasts with professions such as writers, researchers, engineers, movie actors, YouTubers, or international journalists. These roles generate added value that can be consumed by an unlimited number of individuals, regardless of population size. It is worth noting that this concept is not solely tied to the complexity of work; roles that collaborate closely with these professionals, such as makeup artists working with actors or lab technicians assisting researchers, also fall into this category.
In reality, classifying people into these two distinct categories is likely an oversimplification, and the lines between them may be blurred, with many shades of grey in between. Nevertheless, I am intrigued by the possibility of determining an approximation for the proportion of work or workers that must scale with population size. Do you know a name for a metric similar to this or any estimates of what it could be and how has it evolved over time?