I say the beginning of the end was when the millenials were in enough positions of power to utilize the internet for PR. I feel like in North America, the Obama campaign in 2008 opened a lot of eyes to the power of the internet. Maybe his second term more so. Certainly was a tirefire by the end of his run (Trump).
I remember the Nexus 4 / Nexus 7 era to really change how financially accessible it felt to get online with consumption-oriented devices that weren't painful performance-wise.
But I guess for the "normies" it was not just another forum. It was the internet. The internet to them was just facebook and twitter.
People also started using terms like "Social Media" as if it was a new thing. This made no sense to me because the internet was always social. I used to spend a lot of time in forums and chat room in the 2000s. So like, what are these people talking about?
edit: e.g., https://forum.paradoxplaza.com/forum/forums/europa-universal...
Does need a bit of polish on the getting it to run side though... it was designed as a platform rather than a standalone, so it's hard to set up. But the fundamentals are sound as it's just a PostgreSQL database with a Go API which is documented here https://microcosm-cc.github.io/ and at the moment has a Django Web UI (just calls the API, it has no database) but to make it easier to run I'm very very slowly porting Django to Go so that there'll be a single binary to use.
But the whole forum is web API first, and the web site is just a client. The docs are there to say "anything you see this website doing, you can do it through a client you create"... and it doesn't have to be everything, if all you want to do is list publicly visible new conversations on a different website then it's a single API call that you can do from client side JavaScript.
But... if you do want to create an entirely new application using the forum as a platform for it, i.e. a full blog system with the forum hidden but driving discussions, or an app where the forum is at the heart of it but the app looks like a cyclists ride database oriented around events and a calendar but with discussions per ride... well knock yourself out, the API makes that pretty easy and there are apps that do both of those things (though natch, they're closed source and the rides one charges over $200 per year per user for it).
Also, didn't vBulletin have a lot of spam and security issues? It's kind of like WordPress in that using it is nice but managing it is hell.
XenForo is where it's at. Never used IPB but heard they're slipping.