Technland has announced that come February 9, you won't be able to buy vanilla Dying Light digitally. Instead, it will be removed entirely, and replaced with the Enhanced Edition of the game on February 12.
This transition will happen instantly in the US on the February 9 launch date, but European users will have to go without it for three days. Discs are shipping now though, and "should" be readily available if you decide to go that route. Techland is noting that it is phasing out the old game, so if you want it at a discount (rather than paying full price for Enhanced), you should act within the next week or so.