Eh conspiracies on /r/piano, that's refreshing. I don't think anyone here who supports getting a teacher in general is calling it dangerous otherwise. As user CutexAngel puts it in the other comment, "Dangerous is a strong word. It's not dangerous to have fun and enjoy yourself." We can all fundamentally agree on that.

Advising against "ill advice" is not 'vested interest,' and ironically using a term like that is vested interest in itself to promote your own beliefs. It's simply objectively not preferable for your common Redditor to go the no-teacher route. Who knows if you're better at learning than everyone else, or a genius at figuring out how to teach yourself in a manner than sets you on course to do everything correctly? We're certainly not assuming the extremes, and getting a teacher is your best bet.

Don't get me wrong however, quoting most of your words, I think it is irresponsible "to discourage anyone from self-learning and discovery." I also think it is irresponsible to give throwaway encouragement for people to learn by themselves without making sure they won't be trapped doing the wrong thing, or specifically, no regards to their individual abilities. For example, if the fact that wrong posture could lead to physical injuries never comes to their attention as a self-learner, when it could easily be avoided in the first place with the guidance of a teacher, then who is to blame for their injuries? Themselves for not knowing the subject well enough to find suitable self-learning resources? The random Internet stranger with the throwaway remark supporting an ideal without explaining how they should actually go about self-learning? A remark that they may or may not have taken to heart, and thus a remark that may or may not be responsible in part for the injury depending on the flavor of morals you're running with. Or maybe there is no-one to blame. It's nobody's fault, it's simply an unfortunately incident where the learner does not understand their own capacity. In any case, couldn't you have at least mentioned, along with your encouragement, "Posture is important, make sure you get it right, perhaps by looking at videos, or reading teach-yourself-piano books." Would this be the only gift you have for them before they set off on their own? Why this and not other tips and advice for things they may play wrong for years to come potentially, if they didn't know what there were to be fixed? Will you do that for free? Or how much should they pay you for the list of advice you deem important so that you're not morally conflicted? Are you going to be their teacher, paid or otherwise?

I know I sound really sarcastic, but I'm genuinely curious.