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Why are all services centralised?
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A server is measured by its availability. To read content from it, it has to be available. While everyone could use a dedicated always-on device, thats not really the situation you are describing. So you would run the software on your phone/notebook/PC. Those are not always available on the internet.
If you post something to your 50 recipients (as you stated in another comment), they can only read it, if both of you are online. For asynchronous communication (like a post typically is), that is a lot to ask. Especially, if you want to have features like comments, that all your recipients can see.
If you move away from the typical social media principle, then you can have truly decentralized messengers. I think, that already exists.
Another problem with any decentralized approach is how to handle content moderation. It is necessary, since humans are messy. The more decentralized a system is, the harder it becomes to moderate. For example: How would you stop CSAM to be proliferated through it? Or even stop it to being send to you? If the software has some automatic replication feature to enhance availability, then this problem gets worse. Lemmy has had its own attacks with CSAM being posted to many servers. That can get you into legal trouble very fast.