I think it's a bit unfair to mainly focus on the API surface while ignoring a lot of internal details. Linus has talked about how Linux will often have massive amounts of internal code change between releases, while the API surface stays the same. So yeah, the API might be old and crufty, but it's also been around for 30 years. And as long as the API is sufficient, it's the internals that are a lot more interesting. Radically redesigning the API every few years would just accelerate towards XKCD 927, because of backwards compatibility.