> Python itself doesn't do that
The type syntax is python. MyPy is part of Python. It's maintained by the python foundation. Mypy is not part of CPython because modularity is good, the same way that ANSI C doesn't compile anything, that's what gcc, clang, etc are for.
Mojo is literally exactly the same way, the types are optional, and the tooling handles type checking and compilation.
No, because in Mojo, type checking is part of the language specification: you need no external tool for that. Python defines a syntax that can be used for type checking, but you need an external tool to do that. GCC does type checking because it's defined in the language specification. You would have a situation analogous to Python only if you needed GCC + some other tool for type checking. This isn't the case.
It's honestly winning the long-term war because traditional languages have really screwed things up with infinite and contrived language constructs and attempts just to satisfy some "language spec" and "compiler", whilst still trying to be expressive enough for what developers need and want to do safely. Python side-stepped all of that, has the perfect mix of type-checking and amazing "expressibility", and is currently proving that it's the way forward with no stopping it.
This said, if most people use type hints and the proper tooling to enforce type checking, I would say this would be a good reason to properly integrate (optional) static typing in the language: it shows that most programmers like static typing. The problem I focused on in my example isn't the only advantage of a type system.