If your site has auto-playing sound, would you be opposed to chrome adding the speaker indicator on the tab to alert users that your tab is causing the sound?
So in order to show an alert whenever sites that should be encrypted aren't, you just have to show an alert all the time. The SSL everywhere movement and Let's Encrypt are about making encryption easy enough for sites like yours that it's practical to do that.
Basically, your site being encrypted, even if it doesn't specifically need to be, helps to improve security of the web as a whole.