It's Impossible to prove or to debunk (like the existence of God)
I agree, sort of. I think of destiny as being just a way to think about time. If you believe in destiny, you believe that the future is fixed (just like the past.) So you can theorize about what might happen in the future, but "possibilities" are not real things, just reflections of ignorance about the future. In this model there is one unified universe where the past and future are just as certain and real as the present... only the limits of our consciousness make it seem otherwise.
So basically, one model holds that there is one actual future which already exists, and the other that there are many possible futures, one of which will come to exist as time passes. These two views aren't contradictory, they are just different angles on the same fact.
that's what always confused me. No matter what happens it can be written off as destiny and no one can prove it wrong short of going back in time and doing things a different way.
I think this impossibility itself is the proof of destiny. You can't just go back in time and change the course of history because there is only one past. Similarly, eventually the future will be the past, so there can be only one of them.
Which perspective you use doesn't influence the answer to the paradox either. The fatalistic man is wrong in both cases. He does have a say in his future; if he goes to the doctor he's more likely to get well. That's just common sense.