The set of keys for all your customers should be only a few megabytes, so is much much easier to back up more frequently and more copies of.
It's also easier to have a "we do daily backups and delete any backups after 21 days" policy.
Store Invoices, billing, etc, separately. These also usually have a fixed pre-determined retaining policy time. Past that time? Anonymize/aggregate or just delete it.
This extends to criminal law too, for example with statutes of limitation. If, 20 years and 1 day ago, there was a 20-year statute of limitations for murder charges, you cannot be charged for committing a murder on that day (barring extraneous circumstances, such as tolling); even if a law was passed the day after said murder removing this limitation.