Why are people still building systems on top of a language that continually undergoes fundamental changes nearly 40 years after release? Is this not the strongest indication that this language is not well designed, it is unstable, and encounters many issues that flat out don't exist in other high level languages?
What language that is actually used 40 years after release isn't undergoing big, fundamental changes?
Java? Nope, you're getting a fundamental change in Valhalla C++? Nope, new language edition every few years with fundamental changes C? C23 has a number of fairly fundamental changes, expect more in the next language revision
I think your sense of causality is backwards here. These languages are getting fundamental changes because they're being widely used. That is what motivates and drives the change. Languages with no users don't need to change.
As you say, any widely used language gets fundamental changes from time to time.
But most such languages handle much better the compatibility with legacy applications.
Python is the main culprit in most cases when I see conflicts between various software packages that insist to use only a specific version of their dependencies. This is why I have to keep installed many versions of Python, and the Linux distribution that I use must take care to prevent interference between those Python versions.