Before discord, people would just go to steam, facebook, teamspeak, msn, etc. Some platforms are simply popular for chatting, and therefor people prefer going there. Discord on that has a reputation that is game related, and splits each game in that quite well. So even on that im not surprised either.
My biggest issue with discord is that it hogs resources and is highly unreliable for gaming in which you are forced to communicate with your team quickly. Its voice chat has massive delays, audio quality issues (too often i hear people going robot), and common enough even has downtime issues in which voice doesnt work at all (and this is typicaly during times in which people game). At some point my group already decided to go back to teamspeak, which on that did have a few times that it went down, but unlike discord this was only for like 15 minutes instead of 2 hours (and in 2 years only twice where discord was monthly). And yes, you can pay, but if the f2p chat has such severe issues with stability, im not going to risk paying for that.
Discord is good for normal chat however because of its interface. But outside of that, its not good. Even though i still prefer steam on that, but thats because most of my chat is not group based. Steam's group chat is still significantly worse than discord.
For casual PvE gaming however, even if voice chat is included, it does a decent enough job as in that case delays dont matter much. For PvP or difficult Coop however, i would suggest not using discord.
Hating discord because people abuse that, thats not a valid excuse. Hate the people for that. Otherwise you could just hate steam because people are also sending the same trash through steam. Discord started out as a good platform, but feature bloat simply made the things that originaly made it good, function a lot less effective. And even with those issues, its still the best platform around. Simply because its not tied to any game developer itself. The closest to this usefullness would be reddit, except reddit lacks private groups in it, and has a horrible moderation system (echo chamber effect). But reddit on that is better in public availability.
But in the end, use the tools for what they are ment, and on that, for general chat in gaming (especialy acros the globe), discord does its job well.