I mean from what I can tell a lot of its recent success is simply because Slack is terrible. Most of the discord "servers" I use which aren't actually gaming related would be almost the same on slack, but slack has a less generous free tier.
I don't think discord has much of a real network effect, it's just a good value proposition. When the screws tighten that may change.
Slack's UI sucks for multiple different "server"s. Good for work, but no gamer wants to juggle around the Slack UI for 2/3 different game servers, not to mention DMs being isolated. Imagine DMing the same person with 2 or 3 completely separate chat histories, depending on the source server. Many people have multiple servers that share a subset of people, and talking to them would be absolutely insane, not to mention the amount of people that primarily use Discord for DMs that don't care about any server at all.
I don't think discord has much of a real network effect, it's just a good value proposition. When the screws tighten that may change.