When my university lab computer got trumpet winsock and Netscape installed, it wasn't mainstream. When Hotbot was launched and could actually search with some degree of efficiency it wasn't mainstream. When I learned html and javascript from reading Webmonkey posts it wasn't mainstream. When I switched to this new upstart called Google it wasn't mainstream. When I could use Google Maps to zoom in on the house I used to live in it wasn't mainstream. When I could publish posts on Blogger it wasn't mainstream. When I could post on Twitter and Facebook it was starting to feel like it was mainstream. But when my parents could figure out how to get on Facebook on their mobile phones, I knew that the web had become mainstream, even thought it's debatable how much of the web is really left.
In the same way that today's web users don't care about html, http, or urls, digital currency won't be mainstream until they don't have to care about private keys, bitcoin addresses, or even (gasp!) the blockchain. We're a long way off.
EDIT: Since I get the sense a lot of cryptocurrency proponents are taking this as a dismissal, let me say that I do think it has a bright future. I just think there's a lot of work ahead until we get there.