I agree, mostly that. This isn't a matter of greed or maximizing profits either; the developers are people doing a job and need to get paid.
To be honest I'm surprised at the amount of content and regular support they're able to put out, I would have thought the revenue wouldn't be enough to cover all that. Or perhaps they're subsidized by revenue from other products or the parent company.
Another thing to keep in mind is that building something professionally is significantly more work than doing it as a fan project. The fan project just needs to work in the majority of use cases, it can be clunky, have a complicated installation process, and fail every once in a while.
From a professional product, people expect something that works out of the box, is easy to use, and robust for all the weird edge cases people will come up with. If something breaks users will expect a fix, whereas with a fan project they're more likely to be grateful for what they have and accept that it has its quirks. So building this kind of thing is not a one-time effort, it needs ongoing support and regular testing to make sure it keeps working.
Nowadays, players become more and more dumb as time goes on and they come to the point where they care nothing about how a content is completed or polished as long as it is tagged as BETA or whatever. De Dev can add extremely rough speculation mod as official mod(not base game) named something like “Speculation mod - beta”.