For our media website, it took us one year before we could move to HTTPS, mainly because of the AdServers which wasn't ready. We had to talk to each ad provider in order to ask them to upgrade to HTTPS. Unfortunately as media companies have most of their revenue from ads you just can't switch like that.

Also, features that are now available once the HTTPS by default on a website, is the AMP, Notifications, HTTP2 (faster loading) and, of course, the little advantage that Google gives to the HTTPS websites.