Python is the second best language for everything. Having one language that does it all is better than learning several that might do it a little bit better.
JavaScript is very much not the second best language for anything.
JavaScript came about because it was the only choice in the context for which it was designed, and then it metasticized into other contexts because devs that used it got Stockholm syndrome.
Speed is a serious problem in Python though. Python has its use cases, and so do other languages. Things would not end well if we started using Python for everything.
If I wanted to write a 3D game engine, I wouldn't use Python either. But there's zero chance of me ever doing that. For 90% of things 90% of people do, Python works just fine. And the performance thing is actively being worked on and getting better all the time.
Python is the best "glue" language I've ever used. When you want to chain together your program's high-level logic and all of the loops happen inside lower-level languages like Rust, Go, Zig, D or C, Python's performance is perfectly adequate and it's so clear and concise it reads like pseudocode.
Python is secretly a functional-paradigm language. If you're not making liberal use of comprehensions instead of loops (especially loops with LBYL conditions in them), you're doing it wrong.