But wouldn't it be great if you could just write your libraries in Python
Everybody obviously wants that. The question is are you willing to lose what you have in order to hopefully, eventually, get there. If Python 3 development stopped and Python 4 came out tomorrow and was 5x faster than python 3 and a promise of being 50-100x faster in the future, but you have to rewrite all the libraries that use the C API, it would probably be DOA and kill python. People who want a faster 'almost python' already have several options to choose from, none of which are popular. Or they use Julia.
The reason this approach is so much slower than some of the other 'fast' pythons out there that have come before is that they are making sure you don't have to rewrite a bunch of existing libraries.
That is the problem with all the fast python implementations that have come before. Yes, they're faster than 'normal' python in many benchmarks, but they don't support the entire current ecosystem. For example Instagram's python implementation is blazing fast for doing exactly what Instagram is using python for, but is probably completely useless for what I'm using python for.
Everybody obviously wants that. The question is are you willing to lose what you have in order to hopefully, eventually, get there. If Python 3 development stopped and Python 4 came out tomorrow and was 5x faster than python 3 and a promise of being 50-100x faster in the future, but you have to rewrite all the libraries that use the C API, it would probably be DOA and kill python. People who want a faster 'almost python' already have several options to choose from, none of which are popular. Or they use Julia.