I don't think ripple will replace bitcoin this 2018, bitcoin has already proven itself to the world and anyone knows of it. Ripple, on the other hand, is just starting but a lot of people are gaining interest to put their bucks to ripple. Another thing to consider is that bitcoin is decentralized while ripple is not. so it will more of the people's discretion if what they want to use in the future. Bitcoin currently has a high transaction fee while ripple will make bankers richer.