Well if you're that worried about Dropbox going away and needing access to your passwords in between the point it shuts down and you noticing and setting up a replacement, use webdav with your own server, or hack up an rsync/cron-based concoction that will do roughly what Dropbox does in this context. There are 100's of ways to synchronize a file across computers, it's just that Dropbox is by far the most conveniet at this point in time.