There are two distinct threat levels: 1. the mostly anonymized logs that require a technically difficult crack and would impact many thousands (millions?) of p

There are two distinct threat levels: 1. the mostly anonymized logs that require a technically difficult crack and would impact many thousands (millions?) of people simultaneously. 2. activity that can be tied directly back to individuals by anyone with time on their hands. You're absolutely right that there are servers maintaining logs and such, that aren't accessible, but they fall into #1. And category #1 can happen, the famous cheating website being cracked as a great example. But that was itself relatively exceptional since simply being a user was enough to end in divorce. The Slack message history distinctly falls into #2. It's a far more immediate problem.