Don’t want to sound like I’m proselytizing, but there are other free operating systems not based on Linux: FreeBSD, FreeDOS or ReactOS in example. I wish, I could add GNU/Hurd to the list, but from what I know, its unusable at the moment. Redox is also a new OS written in Rust, but it's not ready yet (I think).
Now, are these real alternatives to a regular Linux based OS? In some cases they are (FreeBSD and their family), but most probably would just use Linux for their PC. They aren't even suited for gaming I guess, the likes of Steam. I was just "Acktually"-ing around that you don't "need" Linux for owning your computer. I mean, you mentioned closed firmware, so technically I was allowed to. :D