I think that it will already be used by everyone in 2020. In 2 years a lot can happen. Because bitcoin is only growing and growing. More people are starting to use bitcoin. And when the world will see what is happening with bitcoin, everyone will buy it.
In just this year alone, a lot has happened. Within the next few years a whole lot more is definitely going to happen. As most countries start adopting it so as not to miss out, so many countries will start seeing it as a way to even develop and enrich their economy as time goes on. All this balls down to one thing, huge growth for bitcoin and that is why we should be optimistic.
The way it is making progress, BTC will actually take much less time that the mentioned one before it goes mainstream. Among all the digital currencies, it is grabbing the most fame and I am not amazed at people getting attracted towards it because bitcoin makes an offer that nobody can reject. Prices are higher than ever and more is being expected to come. Bitcoin is the future of the money.
If other altcoins can increases their value, I think these altcoins still have chance to become famous as Bitcoin at now. So, we can say Bitcoin will be the best cryptocurrency has most users, Ethereum or other altcoins still can overpass any time.