Crikey Arantor, that's given me some food for thought.
Firstly, the official Invision and XF forums cannot be put into the same category as enthusiast sites like this one, because they're using the product to sell and support the product. At XF, I've heard the staff refer to it as a business forum, which sounds right for that site, hence behaviours will be significantly different and the sense of community will have a different feel since the members are mostly customers and admins at that, not enthusiasts like on here. That quick in and out with a query that you describe is about right for those product forums. On the other hand, here we'll spout off about anything.
Whether Invision and XF both go corporate with cloud only products and corporate pricing remains to be seen and of course, I hope very much that this doesn't happen. Unfortunately, Invision is already part way there with its huge $850 price tag for the full product and the staff themselves said that they may drop the self hosted version at some point if that's what it takes to keep the business afloat, but that it's not on the cards yet. Not very reassuring, but I felt it was honest and understandable, at least to a degree.
I agree that the pile-on isn't pretty, eg the way it happens for IC. Sometimes these things are justified, depending on what it's about. Perhaps not with IC, but wanna take a pop a Putin? Be my guest - and bring all your mates too for the biggest pile-on ever!
Regarding those pile-ons, the Forum Rules are strongly anti-bullying, so if I see someone getting bullied in that manner, I'll have something to say about it and the behaviour will stop sharpish, I promise you. I've been on the end of it at other forums and it's annoying as hell, I tell you. Other people, perhaps with mental issues, might go into a depressive funk over it and we can't have that here.
I don't know if you run a forum, or are looking to start, but I think all three main commercial products are contenders at the moment, XF, IC and even vBulletin - it seems to have come on some since I tested it over a year ago, with the latest updates including official support for PHP 8.1 that even XF doesn't have yet (it mostly works ok though, just not certified and the odd errors reported by some members). Research and test vB carefully though for customer service and robustness before buying.
In my opinion, a successful community can be built with most forum software, regardless of capabilities, because as you put it well: "they'll come for the content and stay for the community" and that can be achieved with almost anything that allows comments. However, quality, well designed software with useful features will help to maximise that success as it will make the forum experience so much better. In this respect, while XF could improve and the pacce of development has fallen sharply, it's still pretty good as it is.