I think with the expansion of Facebook and then Discord the groups on forums just migrated to those places.
Being able to make posting footage of things a job also has made a massive contribution to the way the media of it all is consumed and distributed. How many construction updates were just someone taking photos on a random visit rather than being camped out for days on end or a 30 minute video with an asinine presenter and a click bait title?
Also many who used them in the "heyday" have grown up, gotten jobs and lives that take precedent over forum times.
I will say though some things do irk me about modern enthusiasts. Which isn't solely a theme park based issue. The almost cult-like following of certain "influencers" where their word is law, the way in which everything must be amazing or terrible with zero nuance to things.
On the other hand, I do think some places had some very unwelcoming "veterans" and often internal politics/cliques/issues that did not help matters for newcomers or regulars.
As mentioned though, this is not an issue solely in this hobby. There's a lot of forums I used to frequent which are very dead, even with there being constant new stuff happening. Discord is king now I feel of the old forum life.