In business (and outside of business), anyone can be interrupted at any time regardless of Slack. If people are expecting immediate replies 100% of the time, that's a cultural thing, not a Slack thing. It's very common to send a message to someone who's on vacation and get a response a week later, just like with email. Of course _more_ conversation happens in real time on Slack as compared to email, but that's kind of the point. They are fundamentally different types of conversations. The various notification options that slack provides (per-channel per-device type, global pattern notifications, etc) mean you can pretty much avoid being interrupted except when you really want to be. People who get mad that you don't respond to Slacks right away are the same people who get mad that you don't respond to emails right away.
Three notifications states is pretty much the same or better than any other sort of emergency communication. Pagers, text alerts, emails, etc have essentially the same degree of distinction between messages, with the reader having to determine severity from context. A text message from GrubHub is less important to me than one from PagerDuty, but my text messaging system doesn't know that. So it's true that if you had a single channel in slack where people were talking about donuts and server fires you wouldn't be able to tell the difference. But you shouldn't have a channel where people are talking about donuts and server fires.
Slack makes it very clear that channels should be treated as disposable. I agree that they shouldn't be spun up and down as quickly as email subjects/threads, but again we're talking about different forms of communication. Slack is, by definition, _chat_, which means there's a lot more of if than email. For the most part chat is meant to reach some end point, whereas email _is_ the end point. It's rarely valuable to go back and read through a chat transcript on some topic, even if it's nicely delineated. Are there extremely verbose email threads? Yes, but they are equally as useless after the fact.
I think part of the Slack backlash confuses me, because it really just moved where this type of conversation was happening. Across every Slack team I see stats for, the ratio of private to public chats is huge, and private chat was happening before Slack (in IM, Gooogle Chat, etc). And I don't think the way those messages are being treat, or the expectation for replies, is really any different. The new chat that's happening tends to be very low volume.