I've never been much of a long-fic writer, so it is hard for me to say. It seems to me that readers often put out the call of 'I wish there was more; I like long stories' etc., so I'd think the epic would be very popular.
But there is truth to what you say about the instant gratification -- if you can write a drabble and get feedback, why write a longer story? The amount of feedback doesn't increase in proportion to the length of the story. In fact, I would think it might have an inverse value ratio.