I have to be honest, I get paid far more per hour than what I would earn doing mods and investing time in paid-for-content. There is also the responsibility with free-mods is that if I really grow tired of it, I would happily leave my work in the hands of the community, to take and use as they wish. With paid-content, I would be obligation in doing the work even though the benefits of doing such would not be worth it.

TF and Dota are rather bad examples as they are pretty much 'horse-armour' mods. Selling recoloured skins for money, or some random hat. They are also exceptions to the rule because they are cult purchases.

As much as I enjoyed Divinity: Original Sin, it is no where in the same league in terms of popularity and activity as DotA or TF, and it appeals to an audience which is less likely to make impulse buys.

I do modding simply because I am interested in doing it and the reward for doing it is the satisfaction received from being able to do something with your own hands that you are proud of, and seeing others enjoy what you did.

Maybe I am simply too altruistic or not selfish enough to consider selling work. After all, companies do sell 'additional unit skins' and 'music pack' dlc for a couple of dollars, so why couldn't a talented individual simply do the same as them?

There are benefits for and against, but seeing statements from people suggesting: "I would mod with my elite 3D art skills but I want $5 for the model, lol freecontent n00bs gtfo" is really appalling.