Placing an inactive mic does raise questions naturally but shipping updates to hardware is not easy/possible. So you'd want to future proof your hardware throughout its lifetime. Had Google announced in their marketing that there's an inactive mic there, it would've become the only focus by the louder segments of the media. Whether or not it would ever be activated.

Perhaps had Google assistant been more useful, fewer people would've felt so upset. The mic can also be used to detect broken glass, etc.