OP here. Iit's telling that so many people are commenting in this thread about how Slack isn't that bad because you can customize it to be less attention-grabbing. The fact remains that Slack is engineered to be attention-grabbing out of the box. Sure you can tweak it to make it less shouty, but Slack is still a product that is sold for money by a company. It's not in their interests to make it align with your existing workflow if they can upend that workflow entirely. No one's making money from you having a meeting or sending a well thought out email, so I think it's worth being cynical when considering paid alternatives.

I think it's also worth remembering that for hacker type folks like us, customizing software is the norm. But default settings are default for a reason, and they will inevitably end up being the most commonly used. When everyone else in your company expects you to use Slack they way do, i.e. for 8 hours a day with instantaneous reply times, then it becomes hard to push against that and eventually it becomes the status quo