I've heard it said that as a developer you need to put in somewhere close to a hundred hours or more for about an hour of content for players.... assuming you are not making the base game engine and really only adding just content. I admit that the modding community can help a bit in that regard if you are talking about doing stuff like making new items in a game or new "crafting" recipes.
I don't know how hard it would be to extend that out for comparatively easy user mods (meaning it is easy to create those mods), but opening up an API is not as easy as you are suggesting either. I agree it would be a pleasant thing to see being done... but demanding it happens isn't a good thing either. Note that Minecraft was released in 2009 and only just barely got something even resembling a formal official API just this past week (and incomplete at that).
Give Gary and Aaron a break here, as they are really doing a good job under the circumstances. In the meantime, if you really want to see them get more content sooner..... make a donation to their development effort and put in a few bucks/quid into their tip jar. They do deserve it.