Boycotts are generally legal in most jurisdictions (not all though, and there are exceptions in a surprisingly large number of corner cases). Had you said something along the lines: "Of course, following the law is just one part of running a successful business. Businesses that operate under legal but morally questionable practices - from putting out content of little originality and low quality, to exploiting ill-paid labour - may face a market backlash." I would have not called you out; in fact, I would have probably added a like to your post.
However, if you, an influential Ars Magica content creator, say:
"even if you could create a highly-derived but legal work ... you [are] vulnerable to an expression of ill will from the creators in the community" and
"if you rip off someone we like, the German Venice people for example, and they do a tearful YouTube video, your project is still toast regardless of law",
that's a completely different story.
Saying "I've not threatened anyone: I've just pointed out that consumer boycotts exist" sounds eerily similar to saying "you know, accidents happen". Accidents do happen, but if you suggest, even without explicitly stating so, that you have the power to increase their likelihood in case another party takes action that is legal but that you dislike, that starts crossing the threshold into a (veiled) threat.
Since you are specifically considering Australia, I would point you to Section 46 of the Competition and Consumer Act of 2010, that prohibits entities with substantial market power to engage in conduct that has the purpose, effect, or likely effect of substantially lessening competition, and carries a penalty of up to 2.5 million AUD for individuals (potentially much more for companies). In this sense, Australia has one of the lowest bars for "substantial market power" in the world, even lower than the EU; I think that you, Timothy Ferguson would actually meet that bar in relation to the Ars Magica ecosystem. And I think that comments such as those above might be construed as a "conduct that has the purpose, effect, or likely effect of substantially lessening competition". At the same time I would stress that, in my opinion, the likelihood of someone suing you is negligible, because from the numbers I've seen there's so little money involved. Still, it's worth noting that the (il)legality threshold might not be as far off as some people might think.
Finally, I think that at this point I have derailed the OP (that was about an Ars Magica video game) enough, so I won't respond to additional posts on this issue on this thread. I'll be happy to continue the discussion in private though, or on another thread focusing on it.