The phone icon on a cell phone is my favorite anachronism

None of the icons you mentioned is of an object so out-of-usage as a floppy disk. Desk phones, handsets, hanging up the phone, envelopes, books, manila folders, newspapers -- all those physical artifacts and actions are in current use right now, and can be found in any office around the world.

And yet, everyone knows what the floppy disk stands for. ;-)