I think cryptocurrency will go mainstream when/if a couple of things happen. First, it will go mainstream if it is somehow easier for people to use cryptocurrency for something than it is for them to use fiat. For example, I believe that one reason Bitcoin was so popular in China is because it's an easy way to move money out of the country.
Secondly, I think it will go mainstream when people become convinced that it's not a bubble that's going to eat up their hard-earned money. Lots of news sources are calling it a bubble. The higher Bitcoin goes, the more it's called that. So...when will people stop thinking it's a bubble? I'm not sure. Maybe it will take a few years. Or maybe it will happen when a fiat currency has problems, and Bitcoin doesn't.
Thirdly, I think people will start to use crypto when they see others using it, and when HOW to use it is not such a mystery to them. As for how to help it become more mainstream...I think one way it will happen is one brick-and-mortar store at a time. If there are places in the real world (as opposed to the online world) that accept Bitcoin, people will fear it less. They might not start using it, but they will be more open to it, and less inclined to avoid it. This would also be a good opportunity for store owners to explain to them how they can start using it, and what its benefits are; as I said, people will also accept it more when they know an easy way to use it. Right now, if you're interested in using Bitcoin, there's a lot of information out there, but how do you know which method is best? I think that a good way to encourage the use of cryptocurrency is if, for example, when Open Money allows people to use cryptocurrency in apps, it also offers an easy guide for people to start using crypto. What websites to use and trust, for example.