@ossobuco,
I don't think that everyone has to be "nice" at all for forum discussions to remain interesting & engaging.
I've witnessed (& also been involved in) some very hotly debated threads here.
My personal view is that there are limits to "reasonable" posters' tolerance & at some point, when the trolling gets well beyond those limits, they tend to disengage.
Like when the troll/s
themselves become the actual subject or focus of a previously interesting thread, rather than what people were initially interested in discussing.
And if the trolling becomes really prevalent on a particular forum, as Paul Graham suggested it can become the "culture" of that forum, to it's detriment.