I think you got the wrong idea about what the Topic is here.
It's not about building up a forum and taking advantage on free assistance and free plugins as years ago (why that changed is another topic).
It's about having a running forum or starting one, and attrakting users to fill it with activity.
I know (and am part of) more than enough existing forums that offer mutural help and assistance regarding the forum topic, at no cost for the public. Those still existing forums continue to practice that. What's mostly gone in the forums I know, is the wide range of offtopic smalltak that existed besides the topic areas. That has definitely shifted to social media.