"the data can get stale before you finish writing it."
This is for archive storage, not backup. You're making the same mistake as those who put backups in AWS Glacier and those who consider old backup tapes to be an archive. Backups (i.e. recovery points) need to be local and fast as a general rule because you are expecting to need them. Archives need to be protected, complete, and cheap because you are expecting to almost never need them but the data is important enough to keep or regs require it.