Bitcoin exist in the meantime because along with its progress, prosper also a country and its people. Thats my opinion. Bitcoin will be forever because bitcoin can changes the world that why bitcoin will exist in the woman life.
It's uncertain to think the lifespan on bitcoin because it depends upon its users. As long as it's getting the interest it needed and keep on being popular it will never disappear. As long as bitcoin is being supported and keep on being used then it will be valuable and stay with us. Bitcoin needs trust to function.