Forums weren't replaced by discord. Forums were replaced by Reddit. Although you post still stands, I think you're very wrong in thinking about the reasoning why people use one or another platform.
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.