Are you saying your great grand daughter will be able to access your cloud in 2067? I assume every current technological interface will be dead and gone ten times over by then, as will all the companies that control them. The idea the digital storage picture in 2017 will last ten years, let alone 50, is not born out by anything we know about the technology or the commerce behind the technology.
You said "I don't know anyone, personally, who backs up images from their phone to a computer." That was what I replied to.
Dropbox, possibly also other cloud storage services, will copy your phone's pictures both to the cloud and your desktop computer automatically, if you enable that feature.
But your comment about access for future generations should include an important point: Computer and phone manufacturers are encouraging us to lock down our devices with passwords and disk encryption, to protect us from identity theft and abuse of our personal data. A phone password can't easily be bypassed. Disk/device encryption, if properly implemented with a good password/other authentication,
can't be bypassed. My digital photos are currently in one cloud sync service, two cloud backup services and an encrypted hard drive in my laptop. My negatives are in a clamshell binder on my bookshelf, or in a box containing developed uncut film.
Computer stolen? My data are safely locked down, no risk of identity theft, and I can restore from the cloud backups. Major inconvenience, but nothing that can't be undone with a handsome chunk of money and patience.
House burns down? Roof springs a leak? Computer destroyed, negatives destroyed. Computer can be replaced and data restored, negatives can't (except for those few I have scanned).
I die? Negatives are safe and accessible, digital photos are safely stored and inaccessible on the encrypted disk and in the cloud (until payment is overdue, at which point the for-pay cloud services will delete my accounts).
What I should do, if I have time: Write down disk/backup passwords and store in a "safe" place, if one exists. Scan all negatives. Make photos available on a non-encrypted backup disk. When will I have time? Probably never.