Uploaded pictures don't last forever in any practical sense.
Yes, in theory google replicates everything 5-fold etc etc. In order for pictures stored with google to last N years, in any practical sense, you need at least 3 things:
- google has to not botch it and lose them
- google has to remain in the business of retaining them for you
- you need to be willing and able to find them
The first seems likely. The second is iffy. Yes, yes, google and facebook seem to be eternal, now, but they're not. They're both businesses with very very very serious problems with their long term business models, and there's a real possibility that they'll be radically different companies in a decade (or entirely gone). The last is the biggest one, though. All these guys are using, or trying to force you to use, a most recent first model.
It is in their interest to force you to display pictures most recent first, and for everyone to access them most recent first. New stuff has to dominate, in order to keep the eyeballs on the screen, to keep the ads paying the bills. Making it easy to access pictures from last week, or last year, or 10 years ago, is emphatically not a useful thing for google, facebook, flickr, or any other online service to do.
So, in real, practical, terms pictures uploaded and shared are more ephemeral than you might imagine. The useful lifespan is really measured in days or weeks.
This is a complicated issue, off the cuff analyses are easy, common, and completely wrong.