The huge growth, which happened in the end of last year, has to be compensated by heavy falling. it is not possible to grow every time, there must be correction, right? You say BTC fall down from 20k to 7k, but I say BTC grow from 1k in the beginning of 2017 to 7k in the beginning pf 2018/.. so I'm ok with that growth
Actually bitcoin growth is exponential as I look it in a yearly basis that is why for long term investment in bitcoin, its a good idea to look for the trend of the graph to make sure that you will earn just by holding it.