If anyone from Slack is reading this - I desperately want Slack to be the thing people reach for everything in their professional sphere. I want people to be exposed to it when they're working on that side project in college, or when putting something together for a family project. Because Slack's search is far better designed for documentation-style posts... but more importantly, the world needs a tool that feels native to colleagues and collaborators from all different generations. If younger collaborators never experience free Slack and their entire view of professionalism is Discord communities, they'll demand Discord. And Discord, with gaming culture still writ throughout its design system, is a big ask for a non-gamer business professional to take in - Slack is quite correctly designed to be the level of approachable it needs to be. So communities will splinter on age boundaries, and opportunities for collaboration will be lost. Some of them might have even made bridges that would have accomplished tremendous social good. All wiped out in the blink of a little rotating gamepad icon.
It's not too late. You could change the update to say "90 days of history, or the most recent 10k messages, whichever is higher." You could even do this silently so you don't need to walk back your posts. Doing so is at least a first step to preserving Slack as a cultural keystone into the far future in an evolving messaging environment. Please make the right choice.