I think one of the market issues is that RPGs don't have the same short play loop that a board game has.
A board game lasts a few hours. A really long board game might last all day. Then the loop starts over. Every time the loop starts over, you have an opportunity to sell folks stuff: "On this loop, play with Expansion X!" is an easy way to add some variety and excitement, and you have enough loops that people who are hardcore about your game are interested in new stuff pretty early on. Even if your game's been out for a year, you could've had a dedicated table with MANY loops through that play experience.
A tabletop RPG doesn't have a short loop. Its loop runs on the order of years. Even a regular D&D group doesn't run 1-20 in one session. Heck, we're lucky if any group gets through 1-20 before breaking apart due to real life constraints. XP and character advancement are highly dependent on DM whims, and DMs tend to be conservative when awarding XP due to perceived "sweet spots" or trepidation for high-level play (which experience with pre-4e games would bear out).
This makes it difficult to restart the loop. New classes, new races, these things don't get used. If you've been playing in an ongoing game since the game's launch and WotC came out with 9 new classes tomorrow, you wouldn't be able to use any of them until 2016. The number of players playing a race from PotA is probably vanishingly small. You're still on the same game loop you started late last year. You aren't likely to start a new one any time soon. You're not finished with your current story.
Adventure modules have a slightly smaller game loop, though WotC's preference for mega-adventures hurts that. When you make a module that runs levels 3-20 or whatever, that's still going to take years to play through, especially given digressions and the like. Something that's more like a one-level adventure or a one-tier adventure might turn around slightly faster, but it's a smaller book, too, and still doesn't give you the sexy new character options that the fans seem to be demanding.
In the middle of a loop, you're not going to sell much (unless it is directly relevant to that loop).
Were I pondering what to release, I think I would focus on smaller adventures, with a higher turn-around time, and give free "support material" (like PotA) linked to them that focused on one or two unique things that the adventure highlights. Like, publish a boxed set that is a level 1-4 kobold cave, complete with creature cards and pogs and mapping aids and handouts. Publish a player's guide like with PotA that maybe gives a narrowly focused enhancement that features in the adventure - say, a kobold character race and a trapsmith rogue subclass. That's all we need for probably like 6 months. Then do another adventure, maybe a 5-10 orc encampment, similar idea. Make the loops smaller, and you can sell more stuff. Make the stuff super tactile - let me see the blood-stained letter from the dwarf, give me chips that represent treasure, all that stuff sounds juicy for in-person play.
It's more board-game-like, then, but still slower, and with an eye toward making the in-person play experience something really unique.