Many site owners are still deciding if or when they will follow the new SSL standard of using a security certificate and switching to HTTPS.
Possible downside of not switching to HTTPS*
• Visitors may start to decline. As discussions about secure web sites become more popular, visitors may avoid non-secure web sites.
• Browsers are still transitioning but the warnings will get more explicit for ALL pages, not just Credit Card or forms. These warnings may further scare off visitors.
• Google has made statements that secure sites will gain advantage in mobile & desktop SERP. HTTPS is already being displayed for secure sites. Bing & other Search Engines will surely follow.
• Eventually, non-secure websites may be considered unsafe to users and purged from SERP altogether.
• Browser support for HTTP/2 protocol is only for HTTPS websites. This protocol greatly speeds up page loads. If your site is not secure, you will not benefit.
*Possible scenarios, no one knows for sure.