But they (the parents) also have a point. Sure, parents love their kids, but there's no question that one goal is to finally get them out on their own. This is a huge milestone, possibly the biggest of all. Forget what age you can drive a car, or vote, or drink; when you move out of the house, you are an adult, an independent being. You are taking care of yourself at that point. Parents aren't just looking forward to the reduced financial obligations and having a bit more free time and space around the house, it signifies that they've finally done it, they've successfully launched their offspring. To reach and pass that milestone, only to get a phonecall later saying "I need to move back home, it didn't work out" would completely suck.
So yeah, maybe they're bluffing, but what they're really doing is trying to get you to understand that this is not something to be taken lightly. This is not just another thing you can try out, and if it doesn't work, hey no biggie. It is a huge step forward, and you do not want to take such a step and then step back. It completely diminishes the accomplishment, can and should be regarded as a failure, and makes it easier to do next time. I moved out when I was 18. When that particularly housing arrangement fell apart, I had no intention of moving back home; I found some other roommates and we found another place. Then later another. There were days I woke up hungry and went to bed hungry, even though I still had keys to a house with a well-stocked pantry and refrigerator just across town. I was out, and I was doing it myself.
Life is about moving forward. Yeah, there are setbacks, and I'm sure your parents will help you in some way if things get bad, but make sure you have it clear in your head how this is going to play out, and what you'll do if things don't work out the way you think.