I think you’re expecting this PEP to be something that it is not. It is not supposed to be a full solution to dependency hell (which I’m not really sure that there is). It is supposed to just allow a static method to declare dependencies, notably supporting both Python package and non-Python-package dependencies. There are plenty of use cases for when you might want incompatible sets of dependencies, for a simple example consider a graphics library with both a Vulkan and OpenGL backend. You could reasonably argue that you should allow both to coexist and just select the best one at runtime, but when you’re dealing with native libraries that’s not always possible, and there is no way to make a guarantee about compatibility without excluding all non-pure Python dependencies.
I think you’re expecting this PEP to be something that it is not. It is not supposed to be a full solution to dependency hell (which I’m not really sure that there is). It is supposed to just allow a static method to declare dependencies, notably supporting both Python package and non-Python-package dependencies. There are plenty of use cases for when you might want incompatible sets of dependencies, for a simple example consider a graphics library with both a Vulkan and OpenGL backend. You could reasonably argue that you should allow both to coexist and just select the best one at runtime, but when you’re dealing with native libraries that’s not always possible, and there is no way to make a guarantee about compatibility without excluding all non-pure Python dependencies.