If you are sitting in the same room with your colleagues, what do you do when there's a power outage? Sometimes there really isn't anything to do but wait. If those kinds of outages were less rare, you'd need to figure something out to smooth productivity. Whether that is buying a generator, or finding things to do regularly that don't require electricity depends on your business.
We have distributed systems that are used by normal applications to solve issues like this. When we need to, we put an ActiveDirectory replica in every satellite office. Frequently organizations forget why they spent the money on so much redundancy in the first place, re-centralize stuff, and it might be years before they experience a big enough outage that reminds them why they were "wasting" those dollars on redundant systems.
As you say, there are plenty of systems that could be made less fragile than they are. Slack is specifically interesting because they made a number of "more central" design decisions, even though chat as a protocol is easier to make "more distributed". What did they get out of this tradeoff? A ton of well packaged features and integrations with other systems.
Could the same thing be recreated in a more decentralized fashion? It is theoretically possible, but it could be years before we see a product/system anywhere near as functional. I would love to see an on-prem Slack product. Atlassian offers the closest suite I've seen that fits this bill. It is not easy to operate these systems at scale.