I would argue the people who will care most about seamless Python interop will only show up, once it's there. That was certainly the case for me when I shortly started using Julia. If it hadn't provided super easy Python interop I would have never cared about any theoretical advantages over Python (well, I discovered Nim not much later and the rest is history...).
Also while you are right that many important libraries in the Python ecosystem are actually written in C/C++, in my experience those libraries tend to be written almost fully embracing the CPython C API (similar to R where big parts are written in R like C). Of course there's also many pure C/C++ libraries which are just wrapped in Python, so I'm not saying you're wrong.
