It is possible to achieve 20k$ but it takes time maybe march or june by 2018 because many people are recovering some money because they used it last year and too many people cash out there bitcoins because the preparation of christmas and new year lasy 2017 therefore if many people cashed out there money last 2017 and they cashed out the same day and the effect of it the bitcoin price will drop. Thats why it decrease price last minute of the year 2017 and 2018 january.