I’m not sure it’d be different if WaniKani were about another aspect of the language. It seems like a not insignificant portion of people learning Japanese are doing so in order to consume content in Japanese, so output may just not be a high priority for some.
On points 2 and 3, I’d say you’re probably right. I would argue that, especially in communicating with other learners, where the field is more even, one shouldn’t feel so much pressure to not make mistakes, but I know it’s not that simple. Although, simply making the mistakes doesn’t really help you improve unless you also realize and correct them somehow, which is where it helps to have someone to point them out to you. In a conversation with other learners, reliable and accurate corrections might be hard to come by.
This also touches on a thought that I had. It may be that more advanced learners have already found other ways to get writing and/or speaking practice—posting on HelloTalk, doing language exchanges, talking with friends, etc.—and don’t have any particular need or interest in using the WaniKani threads for it.
The lack of activity may, itself, even be a factor. “If no one ever posts in this section, what’s the point?”, right?
私もそう思いますけど、「これ、どう言えばいいのか」と考える時間をかけすぎて、まだあまり使いません。I spend enough time looking up words or grammar points and example sentences that use them, running my sentences through DeepL to check that it at least probably says about what I intend, and then through 文法ーCHECK to hopefully catch any glaring grammar mistakes, and back and forth as I make corrections, that it takes a long time to write even a few sentences. I guess a lot of this comes down to a lack of skill and experience with the language, and would certainly improve with more practice, but it does make it difficult to devote the time to get that practice. When posting on forums, etc. I’m also prone to frequently change my mind about what I want to say or how I want to say it, which takes “I’ll just reply to this thread quick.” to “How did I just spend an hour on that!?” more often than I’d like, which carries into my Japanese too, so… That’s a (highly personal) factor as well.
I think both are necessary. Comprehensible input seems to be the true key to improving understanding in terms of passive recognition most comfortably but, if you’ve ever had the experience where you can see a word here on WK or read/hear it in the wild and instantly know what it means, but then you reach for a word to express that concept while producing Japanese and you completely blank on it, you probably get the sense that that’s not enough on its own. You need to practice recalling the language you’ve been exposed to and putting those pieces together to express what you want to say, too. That’s the impression I get, anyway.