There may be some contexts where that's true, but overwhelmingly I've seen Discord replacing forums, particularly for small, closed groups. Wheereas I've never so much as heard of any group adopting a private subreddit! But tons have ended up with a discord as their central social point (whether or not they need or use the voice feature).
I've seen this especially in gaming circles (guilds, clans, etc.) and to a lesser extent in other hobbies or in patient support groups.
I know discord isn't a good match for the technical features of forums, but it seems to be a good fit for how people actually used them.
It was really really awesome.
Technically, reddit and Discourse is more like forums, but a lot of things that would have been a forum 15 years ago are now Discord "servers".
Reddit and Discord piled the dirt on top and added the headstone.
This seems to be fairly common and it seems that as these applications and companies rise and fall that they each bring there own flavor to the mix.
Google maps was concise in 2012, today the app requires a greater number of user inputs to enter a destination for example
Eventually good chat program runs out of runway, gets acquired (wow look at that userbase!) or needs to monetize on their own, and becomes the bad chat program. And the cycle continues.
From my experience there's a split, more "generalist" forums did get "replaced" by reddit, by subs such as technology, games, news, etc. More "in-depth" forums where you expected to see the same guys working on something or having matches together etc I've seen being almost always replaced by discord:fighting games forums(even for games with good MM), dev/security/cracking forums, really anything that isn't "mass appeal".
Discord recently added Forum Channels to try and capture this segment as well.
https://support.discord.com/hc/en-us/articles/6208479917079-...
I think the best approach to a mix of forum and discord-like communities is the app "aether.app" though it had a weird protocol behind it and has been abandoned. But the UI/UX although still perfectible seemed to hit a very nice sweet-spot for me.
The biggest problem came from the fact that we had been all in a discord server in the past that was deleted by the owner after a political disagreement. So in order for that to not happen again we wanted to have a more "community owned" system so that no one would be able to wield such power without some degree of community acceptance.
Sadly, it was not possible to do such a thing on discord as there ALWAYS has to be an owner to the server that's a single individual. Some of these problems can be solved through bots (you can implement very good voting bots) but you're always fighting against the natural structure of the platform, so no solution is ideal.
Well, for me they were never replaced. I still frequent them regularly for some of the best communities and community-based support around (eg. fortunately vendors of my 3D printing and PCB manufacturing equipment host good ones).
I don't think it's ever been sold as like, a general purpose chat platform like IRC or Discord...it's a shame a bunch of OSS projects decided to settle on Slack for their community chat.
Let's think about discord for a minute, why is everyone using it? And what are they using it for? I think the answers here are rather simple, people have a need for communication with peers; this communication usually takes many forms: text, voice, video, gifs, images. Discord is a platform that offers people a very simple and quick way of building communities capable of providing people will all of these forms of communication but really, now a days, every messenger application does the job. So the differentiating factor for discord is the ability to organise these communities better than any other platform (save for slack, which discord modelled itself after). Now, it's not perfect, it's impossible to build horizontal communities on discord but it does the job.
So functionally, discord offers people seeking to build communities pretty much everything they could want. Now, it still has some problems: mainly that it's not very good for long-form or persistent messaging like a forum. And this is why you usually have discord communities with a reddit counterpart, a forum-like website and a real-time interaction community.
This let discord built a very wide userbase, and once you have that then you really get to enjoy the magic sauce: network effects. To put it simply, people use discord because everyone else uses discord and everyone else uses discord because people use discord.
As the internet grows it becomes increasingly difficult to break network effects even if you have an offering that's better. But even then, it's also extremely difficult to be novel given that these big companies can simply acquire you when they start seeing you as a threat (look at adobe buying figma). Companies are quicker than regulators at seeing potential market disruptions and threats, so in most cases they can stay ahead of the game. Imo, a lot of the failings of these companies come from underestimating the competition and then trying to play catch-up.