Is storage really that cheap? I feel like my console and laptop would be very bad off with ⅔ of the storage space dedicated to other people's data.
To answer these questions in the opposite order: both of those examples are places that tend to use quite expensive NVMe storage, because they're optimised for interactive use.
I can buy a 4TB spinning rust disk for under £100 now. Let's say I devote half of that to other people's data and have 2 TB myself, so £100 for 2 TB for the lifetime of a disk (plus power, but connect it to a RPi and the whole thing is going to use under £10/year of electricity). The disks come with a 3-year warranty, so let's assume that they die the day after the warranty expires. So that gives me a cost of £130 for 2 TB for three years. Let's say £120 to make the maths slightly easier, so £20/TB/year. I probably missed something (the cost of the RPi, for example, though they're so cheap they're basically a rounding error), so let's add 50% to that and claim £30/TB/year
Azure and AWS have about the same costs for blob storage. 2 TB on Azure in cool storage (assuming no access, just the cost of storage) is £180/year.
Even if you assume that I want three remote replicas for every local block, so I split my 4 TB disk into 1TB for local use and 3TB for other people, that's still a third the cost of Azure or AWS.
You wouldn't replace cloud storage with a single local disk because:
The local disk has no redundancy and so is a single point of failure.
Off-site replicas are important for protecting against things like theft or house fires.
Perhaps most importantly, the cost of a 1TB disk is only £50. So assume you want 1 TB for your backups, the incremental cost of turning that into a thing that has four-way redundancy with three of the replicas off site is only £50 over the lifetime of the disk. And that's much cheaper than any cloud backup thing.
You just need the hash to assure yourself that your data still exists somewhere in this amorphous cloud system, right
It's not just assuring yourself, it's assuring other people so that if I am hosting 10 blocks for you, someone else will be willing to host 10 blocks for me (or possibly 9 blocks, to account for failure). And possibly you'd also want to mediate bandwidth somehow, so uploading other people's blocks gave you some credit that you could use for storage or uploads.
