Why do you think that? Languages/runtimes with high C integration and fairly exposed C bowels like Python and Ruby have, over time, turned out to be very hard to evolve compatibly.
Because it's a fact? With cpython you can develop things in python if that suits you or C if that suits you. You have more freedom to choose what fits your use case. I'm not saying this is necessarily good, but I don't think it's obviously bad. I'd like to hear from people who think it's the case.
> Languages/runtimes with high C integration and fairly exposed C bowels like Python and Ruby have, over time, turned out to be very hard to evolve compatibly.
That is true, but it's also arguably one of the reasons cpython became so popular in the first place. The ability to write C-extensions when appropriate has been very powerful. It's certainly caused issues, but I think if python didn't have exposed bowels it may never have become nearly as popular. What if numpy wasn't ever written? (This isn't to say that they couldn't have exposed a better C-api with fewer issues, but hindsight is 20/20...)
Why is this a requirement of a "great" language? By not requiring a language to be self-hosting, you are adding more degrees of freedom to your language's design, so I could even see an argument that writing it in C is an improvement. I don't necessarily agree with that, but I don't see why cpython written in C implies it is a bad language. Maybe you could elucidate your thinking?