Many people expect free things to stay the same forever, which obviously isn't realistic, but this is still going to have a damaging effect on their brand. As well with the free tier, there are a ton of OSS projects, community groups, etc. that will no longer be using Slack, and the network effects will drop substantially. This has been seen a bit with Discord, so maybe they are going all-in on dropping these types of customers.
There can't be _that_ many people who are on the free plan that will now be migrating to paid with this change. If anything, it now incentivizes larger groups (which would generate more revenue) to use the free plan since data is gated by time, not by usage. A slack instance with a few thousand active users on the free plan was basically unusable, 10k messages can be eaten up rather quickly. Now if you only care about the last 90 days (which honestly, isn't a bad idea with Slack to not use it as permanent storage) you can have a much larger group on the free plan.
It has no business being the default chat of open source projects. Especially when there's something amazing and fully free like Matrix.
I think a fair amount of their actual sales come from non-tech companies reluctantly paying for it because their developers are pushy about wanting it. Those developers want it because they were exposed to it in some context where it was free.
Lacking that pressure, those companies will just tell their developers to use something they already have...MS Teams or whatever.
If a small opensource dev group gets pissed at slack, and those people have a say in a chat solution at their workplace (paid plans there of course), pissed people won't recommend slack but other alternatives.
My understanding is this sort of strategy wants to archive some sort of network effect so business (which actually pay for services) start adopting a product.
It's meant to kickstart recurring revenue, not to last forever.