I have like 7 Slacks. 2 of them are paid, 5 of them are hobby that I may use once a month. What are you suggesting? That I pay an extra $40/month just so that maybe I need an old message in one of those? I'd agree with you if Slack were indie devs but does Slack need my $40?
The unhappiness comes from the fact that people want to rely on a service, even if it's free. It would make sense if this was a change for the sake of resources or paid user experience. This is for neither, just a tiny push for people to pay or leave. Ironically, It may make me want to stop paying because having all the conversations in one place is nice if I take the hobby slacks elsewhere