The X-Men, Spider-Man and Blade were all handled outside of Marvel and did just fine. In fact you can say Marvel had the advantage of watching people both succeed and fail utterly with their work so they not only knew what to do, but what not to do, which is equally important.

During the Dark Ages, before Marvel wised up and started handling their films in-house, there were a lot of mediocre or downright terrible film adaptations.