Two things happened simultaneously. The forum aficionados were 90s kids who entered middle age around 2010s and sort of weaned away from online engagement, and

Two things happened simultaneously. The forum aficionados were 90s kids who entered middle age around 2010s and sort of weaned away from online engagement, and the new gen had other shiny venues to socialize to begin with. Further, the older folk when they came back for online engagement adopted the new venues. Not everything is cannibalization by evil big tech.