Tens of thousands of users is not actually that large these days for a worldwide chat app though? I don't know exactly how many concurrent users apps like Slack or Discord have, but I'm willing to bet it's at least two orders of magnitude more than ten thousand. Let's not even compare it to the real giants like Whatsapp and WeChat.
Most of those Discord and Slack users are nontechnical and not the target for a developer-focused IRC network. It's like discussing IDEs and then saying "but I bet many more people just use Notepad to write quick notes".