You're really underestimating Discord's free layer. I have a server that's literally just me which I've dumped tons of images and videos into. It's like an unlimited cloud storage bucket, so long as the individual files are below a certain size. It also won't prevent you from seeing stuff after you've got 10k messages.
I think it's a clear difference in business model:
Slack's business model is selling to "the group". The features you have available are what your "admin" decides is good for the group, and upgrades (especially upgrades from the "free layer") need to financed/budgeted as a group decision.
Discord's business model is that individual users are the paying customers so there are fewer features "nickel and dimed" at the group level. If I want better paid features as a user I don't have to convince an entire group to buy into the idea. I especially don't have to convince multiple such groups to do it if I want the same paid features in every group I am a participant in I don't have to convince the admins of multiple groups to pay for them (and/or figure out how to bill for them). Where Discord does have group-level paid features ("boosts"), they are presented as bottom up "collective individual effort as multiplier" more than "the group needs to top down pay a monthly invoice".
From a user perspective, in Slack I'm not the customer, I'm a cog in some wheel that is the customer. On Discord, I'm the customer, and that deeply impacts a lot of how Discord operates including and especially how it treats the "free layer".
