Essentially for something to be decentralised and not ephemeral, everyone needs a copy of the data.

To go into a bit more detail—one of the biggest benefits of decentralised systems is generally redundancy has to be built in otherwise you have a Single Point Of Failure™️, and then you get data loss when it’s gone. Given any sensible decentralised system is designed to avoid this scenario, that data has to be somewhere, and generally the simplest and less expensive (in terms of processing) way to improve on data in one place, is to have it in every place. Any time the data isn’t in one place or every place, you then have an exercise in figuring out where it actually is. This “finding it” processing is going to take time and effort, and if you imagine a standard semi-popular lemmy post, that’s potentially data coming from all sorts of different places, which may or may not be there—this would inevitably make request times ridiculous and basically no one would use it.

At the end of the day, any kind of processing is energy, cost & time expensive, whereas storage makes that part of the process effectively instant and is much cheaper than increasing processing power in both cost and energy.

So basically in this use case and many like it: it makes sense if you’re trying to pick what to optimise, you optimise for lower processing and higher storage requirements rather than vice versa.

The history aspect is more straightforward to understand given the above, if you expect people to care what happened a year ago and want to support that, that data needs to live somewhere

From my experience, how people expect it to work is to be centralized and neutrally hosted, with instances acting as dumb portals, mainframe+terminal style. So it cannot work as people expect and be decentralized.