> I'm just surprised nobody mentioned any decentralised, blockchain-based platform
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.
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]
I'm just surprised nobody mentioned any decentralised, blockchain-based platform where spending time and money building on it is just worth it for the future.
Have a look at the Steem blockchain: social network like publishing and commenting are free and even rewarded with crypto-currency payouts. And as a developer it is very elegant to develop on it.