Threads turn every slack channel into a multi-headed hydra. Now, in addition to keeping up with updates on slack channels, you're also having to keep up with updates to threads in slack channels. Worse than that, if you're looking for an old thread you have to remember the channel it's in and the first message that started it if you want a hope in hell of finding it, since the threaded responses don't (normally) show up in the channel itself.