Subinterpreters for Python
I feel like Lua would have gotten more traction in the early days, and maybe eclipsed Python, if it wasn't so hyper-focused on being a tiny embed-able language/runtime.  It was so well-designed from the very beginning.Every new release of Python seems like it gets bigger and bigger and more and more incomprehensible.  The Python 2 -> 3 transition was a (necessary?) disaster.  And now we're trying, in earnest, to figure out how to get rid of the GIL.  The async/await syntax is a whole other fiasco.  Now we have colored functions all over the place.  Python code doesn't even work with Python code.  It's just an absolute mess.