While it's the first work I'm reading on this topic, I'd want to ask, why so much interest is focused on miner's earnings compared to the core developers?
As some of the users have mentioned above, FOSS comes out of passion and a strong motivation to support the initiative. The developers contributing to the project know what they're signing up for and their work would not necessarily be compensated. However, there are other benefits of being a core Bitcoin dev that devs can capitalize on. For instance, any distributed networking/Blockchain company would get a Core Dev board of their founding/advisory team which would pay the devs in hefty amounts. If the funds are directly not coming from the project sponsors, Core devs have a huge demand outside of the project to make profits from. Its up to the dev how much they want to dedicate themselves outside of the project.