Yes, you can earn decent income through forum if you also put an adsense ads on this. Example of this is DP, it's in 8th top adsense earners. See this post.
Definitely, you may earn good money from forum sites with quality posters. It should generate quality traffic. The topics and sub forums of that site should have keyword generating capabilities.
I think you can earn good amount of money from your forum if you invite good members and pay them to contribute some quality posts. As you good and key word based posts attract more traffic which are then converted into cash.
why would anyone on DP click on adsense ads? Nobody here is actually interested in them directly. So do they just click to support the website? Everyone knows what theses ads are because they themselves are trying to make a $1 a day.
If you can commit three years of hard work to the minimum you could expect something. I am running a growing webmaster forum. We cover the expenses ok. But earnings are far from quitting my other jobs