While I like the idea of starting a hobby OS, but the most despairing thing about it that to be useful it would always have to implement 'legacy' interfaces of other OSes. That is, it cannot stand on its own and create a new ecosystem, instead it has to interface to the world and implement TCP, POSIX, know formats widely used files and such.
You end up with an OS kernel that talks Linux/Win32 and takes on a lot of compat code, protocols, and other paradigms.
I wonder what a hobby OS would have looked like it if it assumed nothing, that is, as a thought experiment, as if aliens on another planet invented computing and started writing OSes from scratch. Imagine we discover software from another planet that would not even work with 8-bit bytes, for instance.
Rob Pike of Plan 9 and Go fame lamented this back in 2000 in his “Systems Software Research is Irrelevant” talk:
http://herpolhode.com/rob/utah2000.pdf
I’d love to see a new operating system that explores radically different APIs for applications. The trouble is writing an operating system is a large effort. Barring market effects, OS has to be heads-and-shoulders better than existing ones in order to convince application developers to write software for it. Windows, macOS, and Linux are good enough for most people, even techies. Additionally, it is often easier to modify an existing operating system such as Linux than to go through the trouble of writing a brand new operating system.