Honestly I don't like the idea of "I'm writing this book/comic in the hopes of it being a film eventually" either. Or even "I'm writing this book so that it can become a comic." If you like the screen so much, write scripts.
I used to just write prose, but adjusting to comic scripts taught me just how much changing the medium can affect a story. Not just the framing or the pacing or cutting things out, entire arcs and character choices that wouldn't work in one or the other. Even though I'm writing a novel and a comic using the same characters! So my gut tells me that if it's that simple to adapt one to the other, maybe I'm not taking full advantage of my medium like I could be.
Of course there are plenty of successful movie adaptations of source materials, some I consider BETTER than the original. But one of my favorite things in comic is layering scenes together in a way that a film can't do gracefully, other than using clumsy voice-over or a lot of jarring cutaways. And if when I was writing the comic I was thinking too hard about how a movie would do it, I'd probably leave those things out, and miss out on that fun and unique aspect of comics.
I feel like it wouldn't be hard to make an interesting movie out of gangsters and magic, but I agree with whoever you quoted, in that I don't consider the work I'm doing now to be a stepping stone to something "bigger."
Though I do enjoy fan-casting characters >.>