Well, they introduce this algorithm to eliminate webpages that are not giving good user interface on mobile devices, but still they rank higher. Before they punish them, they already send notification to Google Webmaster Tools - Mobile Usability
If webmaster fix the errors in Google Webmaster Tools by making mobile layout or responsive design, then they will rank same on Google, otherwise they will punished by this algorithm.
This algorithm is to punish website that is not mobile friendly, not for boosting traffic who have mobile friendly site, and they pass the mobile friendly test.
Just checkout this awesome FAQ on Mobile Friendly algorithm