Who said anything about copying databases???
First, every public forum is crawled by archive.org. You can browse copies there, unless robots.txt on the server blocked it.
Second, the point I made was that Discord and similar stuff are chat-like. What you post there doesn't stay. You don't have threads or any other means to find info that was posted 10 years ago. With a forum, you have.
Discord does have threads now, and they can be saved for posterity (as long as the service remains active). Pinned comments for beginnings of a discussion are a thing as well, but require moderator intervention to note that something worth saving is being discussed. The search function on Discord is quite impressive, but tends to have a poor signal to noise ratio between relevant information and shitposting. However, generally, the biggest failure is that the platform is still missing out on external archiving. Who knows how long the software will even last before the next big thing arrives and all that history is lost.
Historically, I was an IRC guy. Die hard until about 10-12 years ago. Visited many social platforms along the way. This forum is one I don't use too often anymore, as my brain has become accustomed to more "instant" conversation, answers, and gratification. I'm fully aware of that implication, and try to remain mindful about dopamine addiction
I don't think the retro/vintage community is stagnating or shrinking - my twitter feed shows that it's quite alive and well - thousands of people posting their project logs in short-form threads. That platform really needs an easier way to search and find relevant information as well. Twitter is my primary method of choice for documenting my projects, builds, and troubleshooting efforts these days. If you build your network around the retro scene, there are a plethora of helpful people and the content is excellent usually.
Forums overall do seem to be an excellent method of documentation that are relatively easy to search and archive, but my guess as to why forums as a whole are failing is due to the same issue I mentioned above - they're slow. People don't get those instant answers when they post a question that hasn't been asked before (or was not easily found via search). With the speed of communications these days, many folks desire the quickest option. That ends up being Reddit, or Vogons, Discord, Twitter, or *shudder* - Facebook. Right or wrong, it seems to be the way humanity as a whole is trending. Not much to be done there but wave our canes and yell at the passing clouds.