I can only think the reason comic writers or novelist haven't made the jump to writing screenplays is because they are not allowed, not because they don't have the ability. To be honest writing screenplays is a lot easier than writing a book. In this age of specialization you are generally allowed to only do one thing until you make it big then you are allowed to do whatever you want.
Doesn't Steve Martin write both books and screenplays as well as anything else he wants?
Why would be the pressure be on Spiderman 4? Spiderman 3 made almost 900 million worldwide. Does TDK have another 300 million in it?