Have you seen Genode (1) ? An operating system framework with a pretty usable OS built on top. Last I heard, it was getting pretty close to being usable as a daily driver. lots of cool tech (micro kernel(IIRC), capabilities, sandboxing as a first class citizen, GUI system, posix compatibility layer, etc). Its been around for ages, has full time developers (its used as the basis for some (all?) of their products.

From the website: "Genode is based on a recursive system structure. Each program runs in a dedicated sandbox and gets granted only those access rights and resources that are needed for its specific purpose. Programs can create and manage sub-sandboxes out of their own resources, thereby forming hierarchies where policies can be applied at each level. The framework provides mechanisms to let programs communicate with each other and trade their resources, but only in strictly-defined manners. Thanks to this rigid regime, the attack surface of security-critical functions can be reduced by orders of magnitude compared to contemporary operating systems."

1 https://genode.org/

The true rite of passage for the child hacker I remember my dad and brother taking a floppy to copy a sys file to restore a win 3.1 install from the Sam’s display computer in the pre-internet days