> why hasn't a better, more general icon representing the concept of persisting something emerged?
Icons are good at representing objects but terrible at representing behavior over time unless it's simple directional movement. How would you symbolize something like the concept of persistence? "Data that can be retrieved unchanged in the future"? A pen, a clock with progression arrows on the hands, and reading glasses? Surely not if there's already an established convention available. The floppy disk happens to be that convention, as the last object that was unambiguously connected to local storage (an optical disk would be ambiguous for recorded music consumption, HDD were supposed to be invisible to the regular user and USB sticks could just as well be any of a variety of radio interfaces in the same form factor). So floppy disk it is. And it's not a problem at all that the convention, once established, is now used by generations that have never seen a floppy, it's just visual language to them. Complaining about the lack of connect between that language and the life realities of it's users seems to me as pointless as complaining that the word dog does not sound like barking at all (why not whoofwhoof?) or that the word hammer does not sound like the noise of hammering (why not banger?).
And still, the floppy disk icon is in decline, because applications that eschew local storage tend to use the "arrow up to cloud" icon. Another convention that has absolutely nothing to do with what is actually going on (few datacenters are actually hidden in clouds)