It's possible to negotiate bulk pricing. The billing is also based on the number of active days per user: https://slack.com/help/articles/218915077-Slacks-Fair-Billin..., so a 5000 member community doesn't necessarily mean they'll have to pay $7.25*5000 every month. But agreed Slack is generally a poor choice to host any non-profit-making organizations due to having to choose between either gated knowledge or allocating a hefty expense.

Discord is the better alternative for now due to the lack of content gating in the free tier but that might change when they become desperate enough with their profitability push.