During the early 2000's you could put up a forum and wave the magic wand and presto you had a forum and you had forum members. And I can tell you from a user's perspective 90% of the forums that I have found over the years, I have found them by google searches. And 10% were recommended to me by other users of the forum.
Fast forward to 2017 and 99.99% of google searches are going to bring up blogs, social media, youtube, and google itself. (When google shows a excerpt of a website) I did leave that 0.01% chance in there for when people search "Forum" or every blue moon google decides to throw a forum post in the search results.
Which makes me believe deep down in my heart if forums are going to survive, they need to be 100% content driven by a CMS. What do I mean by content driven by a CMS? You need something like wordpress on the front end and well thought out and written articles to feed google search results to drive people to your forum. And then have a discuss in our community link to drive them to the forum.
And even then they are most likely not going to sign up, but sooner or later someone will. And it will be a trickle down effect as you write more articles you get more members.
P.S. these are just my thoughts take them with a grain of salt.