It would not reduce code quality if used in instances where it replaces return nil, err - which is what it is intended for. It's not intended to replace wrapped errors, or error handling, it's intended to replace 3 lines of boilerplate with one word - in the cases where you want to push the error one level up the call stack.
Having used go extensively for the last 5 years and built some large projects with it, I disagree this is not useful. I think it's a good proposal in the spirit of go and should be adopted.
I'd prefer this as a keyword myself, but of the 6 million instances of if err != nil on github, I think around half could be eliminated by this pattern, so I think it would be useful, if not overused.
https://github.com/search?l=Go&q=if+err+%21%3D+nil+%7B&type=...