Why would one even bother with that when you have like... a cable at hand and disk space.

Though recently simple things as copying files have been a challenge.

You still need to back up your disk somewhere right? Disks age and fail.

Related, but why phone disks don’t age and fail? Simply because of low r/w?

I think they do, but my guess is that most people don't keep their phones long enough for that to be the reason to stop using the phone. Usually the phones get damaged/lost/too slow to run modern apps etc.

You backup the disk to more disks. Isn't that how it's done?

There are differences between iCloud and local backup. For example, iCloud backup doesn’t backup your Notes or Health data but local backup can. Sync != backup — without a backup, if you sync a bad change you’ve lost the data.

https://support.apple.com/en-us/HT204136

Both of these are stored in iCloud (if you enable) and are backed up in the cloud (and E2EE). All other large repositories of data are not sourced from my Apple devices

It is only E2EE if advanced Advanced Data Protection for iCloud is enabled.