I don’t think it’s going to be an incentive at all really.
It’s already been released that modders only get 25% of the revenue of the mods they put up. A LOT of time can go into making a mod, and most of the time people share it as a point of enjoyment to the game/title. If you’re going to make them professionally, 25% means you’re going to make it cost a lot to cover your losses, or just get a slap in the face with every sale. Also, the creater of the mods does not get to remove their content from steam if it ends up being illegal or otherwise problematic. There’s some great forum posts about it.
I for one hope that it encourages people to make great mods… But people were creating great mods before steam… But I don’t think the developer get’s any kick-back for modders making anything for their game except for the continued interest in their title (which means nothing unless they use DLC)
There has already been some problems with mods going up that aren’t useful, or even abusive. Some of my favorite were “trash mod” which literally, adds usable trash barrels to the game… or “Almost nothing” mod… which adds almost nothing to the game. Steam isn’t planning on monitoring the usefulness of the mods in steam… So buyer beware.