I think there's a legitimate FOMO involved with not quickly reading messages. I say legitimate because it's not just missing out on inconsequential chatter. Slack is a business tool and business decisions get make over slack. The folks that read every messages and respond quickly are therefore the ones that get to contribute the most to decisions.
When waiting too long to respond, the conversation has already changed and it's too late to contribute. And Slack's threads don't solve that problem IMHO.
You're missing the key modifier: legitimate. It's not an irrational fear of missing out; it is a very real concern about a culture where being less responsive on Slack reduces your influence on key business decisions. "Close the app" is only good advice if you both want to and can afford to opt-out of any involvement in those decisions.
Agreed. There are times I only join conference calls to ensure that no work gets assigned to me without me hearing about it first, or that I have the ability to chime in as work is being assigned. I implemented this policy when it became clear that work was being assigned to me (imo incorrectly) because I wasn't there to say no.
If decisions are being made over Slack, I have to be involved or suddenly I may find myself responsible for something I have no control over. Simply because I was not there to let people know who actually is responsible.
When waiting too long to respond, the conversation has already changed and it's too late to contribute. And Slack's threads don't solve that problem IMHO.