I'm actually curious, how is Slack a time sink? I muted most of my channels and just check for DMs, thread replies and @channels from important channels. It's at most 30 minutes a day, roughly the same as email.
True, but in every chat app ever that I've used, nothing is muted by default.
And the reasoning is quite simple, for the average user who doesn't change any default value, a by-default-muted chat app is useless. There would be no messaging as far as a regular user would be concerned. What good is a chat app where you can't chat? :-)
Maybe others just accept the defaults