I believe that "open core" was not a response to the question of "how do we write open source and still earn a living?" but rather "how do we leverage open source to make a billion dollar company?". I think the distinction, however minor, is important.
I am not sure there are obvious models/examples that are working. I think there's a reason that "open core" is an increasingly popular thing to try to do. I'm not sure if I can think of many earning a living while producing high-complexity high-quality open source software _except_ "open core".
(And I still think "open core" is problematic for the continued viability of actual open source).
Maybe. But I also think that kind of funding for open source you talk about is not there for high-complexity projects in the present time.
It's not that I don't like that traditional model. I am legitimately worried it is no longer feasible.
Or, there are Fair Source License / Commons Clause / Business Source License /... that allow developers to capture some of the value they generate, but this of course means they are not open/free/libre because they limit some of the freedom to accomplish that.
Pick your poison. :)
I find in general it's easier to get free labor on something the newer it is. Both because it's more exciting, and because the more 'mature' something gets the _more work it is_ for _less gain_, there's more time dealing with backwards compat and filing off rough edges and keeping the existing code just working, which is all less interesting to do for free. Plus, the more mature something is, the _more work_ it _takes_ to keep it going.
The question is, who is going to write open source projects at a complexity/polish level similar to ElasticSearch. And how are they going to pay their rent?
People working on it need to pay the rent somehow.
I am suggesting that the way people paid the rent while working on open source historically in the "first phase" of internet open source (doing it on the clock at jobs for which the open source was useful for their job role, but producing the open source was not their job) -- is no longer working to get software written at the complexity level of 2019 software, as software gets much more complex requiring more people coordinating with themselves and spending more hours.
I agree, at least for the sake of argument, that "open core" is distasteful and problematic. I believe it was a response to the question of "how do we write open source and still earn a living." It may not be a satisfactory answer. I'm not sure what other answers exist. If there aren't better answers, open source will shrink, including by "open core" and such.