Magtape is the only viable medium for things which are actually "backups" as that term is understood in the professional IT arena. Every other possible medium for backups has faults which cripple it for one or more of the requirements which backups are required to fulfill -- primarily that's length of storage, but there are lot of other fun failure modes.
Sure, spinning magnetic storage, optical media, and flash drives each have some advantages for specific purposes.
But go pull the post-close EOY General Journal from 1996 off of one, I dare you.
And if you think that's an overly strict requirement, a) you're probably wrong, and b) I can come up with lots more that you won't.
My commercial backup guidelines are these:
You need it backed up on at least 4 pieces of media, of at least 3 different types, in at least 2 different cities, in at least 1 different state; bumping each of those numbers up by 1 is not unreasonable.
Only one backup can be on optical media; only one can be on spinning magnetic media, whether it's powered or not (this includes the cloud, and local external HDD backups, whether powered 24/7, alternating, or pulled and shelved).
Flash media is right out, as are SSDs.
I can pull 20 year old DC3000 tapes off my shelf and read them -- as long as I have a SCSI interface for the computer in question.
GNU tar is great that way.