• Ask people to provide optional contact information for an "executor of their estate"—a person who can make decisions about what happens to their data on their behalf if they cannot be reached.
• Ask people for a secondary credit card that can be charged as a backup: specifically, suggest that this be the personal card of Someone Important in the company, who will be likely to notice the charge and flip out.
• Ask for a flat-fee deposit to enable a secondary "long-term storage, no uploads, no monthly billing" mode of usage. Make it enough money to be motivating when you imagine just schlepping this hunk of data around for the rest of your life. If the user has paid this deposit, and their regular card gets declined, switch them to this mode and consume the deposit. If they close their account, refund the deposit if it hasn't been consumed.
And so forth.