The difference between a good pre-made plot and a bad one is often how open it is. Whether the characters are overly defined by one party or not and whether there are available plot hooks for the other player(s) to either easily implement or grab onto. If neither of those are present, it can typically be a poor pre-made plot, though we suppose exceptions can exist to that too.

However, with sufficient amount of hooks and character options, an RP isn't doomed even with the final result of the story set prior to launch.