I think they didn't monetize a bunch of stuff with NWN just because back then, microtransactions was not really a part of gaming. Now though, with microtransactions being the Status Quo, they could monetize the crap out of additional chunks of content. Even if modders are making content for free, the developers could make better-looking, shinier, more polished stuff and it would sell. Just calling something an "official" add-on or module would automatically make it more appealing to a lot of people than just amateur mods. They could sell asset packs, new races/classes/subclasses, their own "official" adventures, cosmetic options for characters, and like I mentioned before, rent out servers for persistent worlds. With how huge D&D is right now, if a true, modern, user-friendly, graphically attractive version of the NWN toolset and DM client came out, it would probably be even more popular than the original NWN was.
Glad to read OP and some posts like this, who has the same idea with me. I also wrote
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=705609&Searchpage=1&Main=92331&Words=%2Bmodding&Search=true#Post705609
Such post about NWN style "DM mod" modding tool.
Let's pray WotC and Larian both are convinced and agreed to develop such tools, and make a good model to protect their copyright and make more $$$. This is win win for players, fans, and developers.
