Is the kernel really the differentiator there? seL4 is a Proven microkernel that is some 9000 lines of C. This ancient SO post[0] claims Linux is 140k lines. The kernel is just a tiny component of the many things required to get an OS up and running. I suspect most projects just peter out as the enormity of the complexity becomes apparent.
I’d love to know what the smallest OS is that can boot on an old MacBook, get to network (wired or wireless), and includes a basic browser (tui or gui) even if it doesn’t drive all the onboard hardware. That feels like it would be an excellent learning platform.
DOS just isn't much of an operating system, it's more like a bunch of helper functions preloaded into RAM, and you're free to do whatever you want with or without them.
[0] https://unix.stackexchange.com/questions/223746/why-is-the-l...