>Google essentially is moving everything to Go ...[snip]... and then module by module will rewrite it in pure Go.

I can see this happening for some core modules sure, but I think you've underestimated the work required to convert the sheer amount of Python code at Google. There is a tool inside Google which graphs the number of lines of each language in Piper; I don't think I can quote numbers from my time there but it would be no small feat, even for Google.