Magnetic tape is an excellent backup method even today. LTO cartridges already have tens of terabytes of capacity. The problem is there are no consumer tape drives, only expensive enterprise models.
Well, there used to be. The early QIC drives that hooked up to a FDC connector like the 3.5" were pretty ubiquitous in the prosumer world. They eventually transitioned to better tapes/standards, but all worked well and even Windows eventually had native support (via Microsoft Backup). I restored my files off of these tapes 25 years after recording them, so I'm grateful they existed at the right moment in time when drives and floppies would have otherwise failed me. I naively assumed in modern times LTO must be dirt cheap due to decades of competition, but it never came to be.
Only the tape drives are extremely expensive (thousands of $).
If you have hundreds of TB of data, LTO, including the cost of the drive, becomes cheaper than HDDs.
The QIC tape cartridges had the problem that the rubber belt which moves the tape disintegrates after a number of years, making them unreadable without special equipment.
LTO is expensive because it's enterprise-oriented, and despite the O in the name meaning "open", seems actually a proprietary standard since you can't just go to their site and download the specs.