Thanks for mentioning the Fediverse.
Who is paying for server costs?
The owner of each instance pays the costs for hosting a given instance. So, basically, domain + hosting.
Note that these costs can be really low; Pleroma[0] can be run on a Raspberry Pi 3 or a $2.50 Vultr instance[1]
What about decentaralized platforms that are not blockchain-based? The Fediverse (a part of which is write.as) achieves this goal of being decentralized without using any kind of blockchain technology.