The high quality of the game itself probably did not leave a lot of time for putting in loads of content, unfortunately. It was sort of a sacrifice, and sure they hoped multiplayer would allow people to basically create their own content to increase play-time - which for many, has worked fairly well.
I would pay for single player DLC. They created an amazing game world that it almost seems sad that won't add missions and some extra story line to it for a DLC product. It almost seems squandered. The next GTA may take 4-5 years and I'm sure people would pay $30 for a series of extra missions in the meantime.