Part of the problem is companies worrying about return on investment - they want to be able to either keep selling new games or at least new expansions. Making a game like NWN at this point would still be doable but it would require a very different system of payment to work - a much higher up-front cost, some sort of subscription model, or something else entirely. It's simply not worth making a user friendly toolset for most games.
Or you could release it on steam (as an option, rather than forcing people to use steam DRM), opening it up for hundreds of people, who might buy it when it comes on sale, or just if they see it in general. They will recommend it to their friends if it is good, who might buy it, and recommend it to their friends, so on and so forth. I think it is worth it. You could have a Steam Workshop, which would keep the game alive, and even might persuade people to buy the game if it has enough good content and modability, because that is a definite selling point I have heard people mention.
Also, if people wanted to make money off their modules, they could send it to the team in charge of the game for sale if it is considered to be of a high enough standard. Now, you might not want this sort of idea taking hold, that people would be able to make money off modules, but, they could sell them for really cheap, or bundle them together in module packs, giving both the developers a small amount of money, and the module maker, encouraging them to make more content, and giving the developers money to make expansions or sequels to the game. Then, hopefully, you will still have people who make modules and content for free, despite the fact that they could monetize it if it is good enough. (You could always just remove this feature and have the team develop modules for sale like the premium modules, while all community modules are free, but if cash is really short, then it could be done I suppose)