> I mean, all that skill, that knowledge, the experience - working on something which, almost by definition, very few people will be able to use.
This sentiment makes me sad. You can do things for fun, y'know? And when someone's just having fun, not hurting anyone, and being creative - that's got to be more than just tragic. It's basically the definition of a hobby.
You can me things for fun, but the language on the website is ridiculously bombastic for that: https://www.getxeneva.com/
I would also assume that a lot of these hobbyists are excellent engineers at their day jobs. While tons of the osdev work is going to be specific to osdev/hardware, tons of it will make you a better engineer.
The operating systems course I took in college was foundational to my understanding of how programs work and the memory model of modern computers.