"If you need a preperation step to drive."
You shouldn't. As a skilled dancer, you must have the ability to generate a full swing by descending through the leg that you are already standing on, without a progressing prep step.
However if you cannot generate an even fuller swing with a progressive prep step (descending onto the leg and then through it), then you don't know how to do a proper prep step that could give you such benefit.
"Then on your next Natural Turn or a Closed Change or Spin Turn or what ever. How are you going to manage without a preperation step."
The whole point of the prep step is to substitute for the missing preceding figure at the start of the dance. It's to give you something to come from, in the situation where there was nothing before. The question is not how you can take a normally full action without a prep step, the question should be how you can start a dance in the middle of what should be the most continuous phase of flow from action to action. And the answer to that question is to use a prep step - which is a sort of "ending of a figure" - to lead into the first real figure.
If you get on a playground swing, do you start at bottom dead center and hope to somehow get going? No, you walk yourself backwards partway up the curve, and release your weight from there. Same idea with the prep step - we release our weight from partway up, so that by the time we pass through the low point we are already cruising right along with energy to carry into the upswing.
Yes, you can start the playground swing from a dead stop at bottom dead center by pumping with your legs alone, and you can start a ballroom figure by pushing with your legs. It's a physical challenge and you can pat yourself on the back when done. But it's not elegant, and it's not dancing.