I believe the second question is crucial here. In many places I worked it "solves" the problem of people who feel they need an answer now and I should stop what

I believe the second question is crucial here. In many places I worked it "solves" the problem of people who feel they need an answer now and I should stop whatever I'm doing now and give them my undistracted attention. In other words, Slack is used to prevent me from working. The main problem here is the expectation of synchronous communication. In email, async is sort of built in - nobody expects my answer 5 seconds after they clicked "Send". With Slack, they get nervous if they have to wait too long.