The typeshed project, which provides PEP 484 type stubs, is currently discussing to distribute non-standard-lib type stubs via pypi (https://github.com/python/typeshed/issues/2491). Currently all stubs are vendored by the type checkers, but this approach doesn’t scale. Similar to DefinitelyTyped in the JavaScript world, we’d like users of a package foo to be able to install the corresponding stubs by just typing pip install types.foo or something similar.
But we’d need to ensure that people can’t squat these names for security reasons. As opposed to other Python packages, people will just try to install the type stubs without previously checking them, and they should be able to. But without namespacing this would open a wide door to attackers. So for us, namespacing is essential.