What incompatible versions of pythons do you mean? I'm entirely unaware of any forks, and the youngest version I have to supply at the moment is 3.9, which is over 5 years old and available in all supported platforms.
Try to run any random python program of moderate dep use on your python 3.9 system interpreter without using containers. Most likely you'll have to use a venv or the like and setup a special version of python just for that application. It's the standard now because system Python can't do it. In practice, pragmatically, there is no Python. Only pythons. And that's not even getting in to the major breakages in point version upgrades or the whole python 2 to 3 language switch.
Try to run any random python program of moderate dep use on your python 3.9 system interpreter without using containers. Most likely you'll have to use a venv or the like and setup a special version of python just for that application. It's the standard now because system Python can't do it. In practice, pragmatically, there is no Python. Only pythons. And that's not even getting in to the major breakages in point version upgrades or the whole python 2 to 3 language switch.