Modern hard disks are extremely reliable, and if you can keep them spinning continuously with nice clean power they'll run for a very long time. The problem of course is keeping them spinning in the face of power outages, PSU failures, system upgrades, etc. In my experience few drives have failed while running, compared to the number that were running fine but then failed after a cold boot (power cycle). Based on this, I suspect your best bet for storage is to build yourself a NAS or a file server using as many drives as you need to hit the storage capacity you need, using RAID1 or RAID10 (avoid RAID4/5, with large drives those are no longer good options). After 3-5 years or so, set up another one (growing capacity as needed) but keep the previous one on-line and use it to back up your new one. 3-5 years later, repeat, setting up a new unit #3, making previous unit #2 your new backup device, and retiring #1 which has by now given you 6-10 years of faithful service. If you're using drives with 3-year warranties, aim for replacements at around the 3 year mark. If you use drives with 5-year warranties, you can wait longer. Try not to buy all your drives at the same time from the same place, spread out your purchases so you'll get drives from different manufacturing lots.
That said, I don't do this myself. My own practice is to run a single large file server for 8-10 years or so (using RAID1 or RAID10 of course), backing it up to tapes which I then store somewhere reasonably far away. I trust tape for backups, but the key here is that they are *backups* of data which is normally available on a file server. I do not use tapes for off-line archival storage, not because I don't trust the tapes themselves but rather because I don't trust the tape drives (and the old software) to continue working, which may put me into a position where I decide I want to read a tape one day but no longer have working hardware/software to read it with. Tape is very durable and easy to transport, hard disks are more fragile.
Regarding the cost of tape, I use LTO4 tapes and drives right now, which are very reasonably priced and hold a lot of data. I typically get between 800 and 1000 GB per tape, so a 'full' backup is 7 tapes (then I use LTO2 tapes for incremental backups). This is getting to be a lot (it was 4 tapes when I started with LTO4) so I'm thinking it's about time to upgrade to LTO6 (LTO8 is the current generation, so bargains on LTO6 stuff should be easy to find). Even so, I'm still spending more on tapes and tape drives than it would cost to buy external USB hard disks. The problem is, I don't trust USB hard disks to actually work when I need them. I have had too many that just fail to power on when I go to access them. My backups need to be stored on something *more* reliable than an internal hard disk because I'll only need to access them if my already very reliable RAID1/RAID10 disk volumes have failed. Since external USB drives have proven themselves to be *less* reliable than internal drives, they are logically a terrible choice for backups.
In a nutshell, I am using disks as my primary storage because they are incredibly economical. I'm also using disks as my primary on-site backups via RAID1/RAID10 (yes I understand these aren't 'technically' backups but relax because); for off-site backups I have another copy of my data on LTO4 tapes which are kept somewhere far enough away that a fire, hurricane, or other disaster is unlikely to destroy both the primary and backup copies of my data. I also have incremental backups on LTO2 tapes which I don't bother to store off-site; those would be used only if I had to recover from data loss due to a malware outbreak, hardware failure, or stupid mistakes.
