I agree with what you're saying, but what you're saying is not how it works at Google. At least the teams I've been at. Design docs aren't a prerequisite to start working, and in many cases a design is nonsensical if you haven't actually at least prototyped what you want. It's just a tool to help you be comprehensive when you want to decide between alternatives, and let other eyeballs help you decide.

"this will never work", without an actual argument, isn't Googley :)

A big design doc is a liability for sure, in the same way as code is.