Theoretically, someone might be able to get away with releasing VPs that would allow FS2 to be played without any retail files, but it would require (among other things) new voice work, sound effects, and interface art, and if the work is higher quality than the original retail stuff, it would likely become more popular than buying the game from official sources (such as GOG), which is always a recipe for a lawsuit. (Companies generally don't care too much as long as people are still buying their product. When a product derived from their IP becomes more popular than the original, though, they tend to flex their right to shut it down.)