Not sure if you can compare fast food to Soylent, because there are many different types of fast food and not all of fast food is bad for you. Fast food can also contain vegetables, live cultures, and various assortment of ingredients which are good for you. Overindulgence of anything is bad, fast food just makes it easier because it's easy to get, generally tastes good and is packed with a ton of calories. If you ate fast food very carefully, then you'd probably be okay.
Furthermore, there probably isn't a simple shortcut to nutrition. If you want to be healthy you need to be involved in what you eat.
I'm sure the driving force for Soylent is profit as well, otherwise they would have done years of testing and scientific studies, before releasing a mix of chemicals as a solution to food. I suspect they'll get sued down the road, there is no way to predict what long term effects this will have when someone decides to use it for years.