>Erlang will need to market itself once you start seeing unemployed Erlang programmers. All marketing Erlang does for Erlang programmers at this point is create competition for them
I think you're getting at the idea that marketing a language stimulates demand for programmers or it stimulates the supply of programmers. If there's a fixed number of positions for Erlang programmers more proficient people to fill those positions will lower wages. However, if the marketing convinces people who have problems to solve, that Erlang and the work Erlang programmers can do is the solution then demand will increase.
However, whereas econ 101 helps identify some of the central concepts, later courses would explore the dynamics in greater detail. For example, languages derive value from their communities and the network effects. Or, someone who is called a Erlang programmer could program in many other languages, but may enjoy Erlang more, or believe they can be more productive if they're able to use Erlang.
Anyway, it makes me think of open source projects where there are difficult people. You can tell non-paying, non-customers they don't have to use the work you've publicly released for free when they find problems, but presumably you're deriving some utility from releasing the software, and your utility function probably isn't served by getting your work ignored.
Even if advocacy only encouraged new programmers rather than marketing the language to people who have problems to solve, I think that advocates for the language want its usage to grow. There are language communities where people talk about proficiency as some sort of secret gold mine, but I think those 'secret' high-paying jobs are probably involve a lot of uninteresting maintenance of legacy code.