I suspect that most folks running phpBB don’t get beyond the 5,000 member mark. Even fewer get beyond the 10,000 member mark, or three years of consistent growth. What happens when you get to that point and can’t afford to run your board anymore? Then it’s entirely possible for a board with over 100,000 members to simply disappear.
I have posted a lot about the Adsense program over the past years, specifically related to advertising on phpBB boards. I currently do not use Adsense, but I did for many years. However I never relied on the revenues from that source to keep my board running. If I had, I might have ended up like this case study:
Warning to Webmasters: It can happen to you
The link contains a case study about Soccerpulse, a web site with over 100,000 members that closed up shop because their Adsense revenues declined and they could no longer afford to run the site.