Interesting question. Those 5.25 disks are generally similar in quality of the newer 3.5 disks. And 3.5 disks are also considered antique since most new computers don't have a floppy drive anymore. Anyway, I have an USB-based 3.5 floppy drive which works just fine.
Looking at my own floppy disk collection from two decades ago, I noticed that most of them have had some damage due to years of not-using-them. Mostly bad sectors or just random blanks. These disks use a magnetized surface and this is slowly leaking away. (Faster if stored in sunlight or near large magnets.) I was able to still get data from some of those disks, though. Maybe about 20% of my disks are still completely readable. I did store them in a dark place, well-protected in a floppy case.
For disks with damaged data you might want to use a disk drive that's more sensitive than regular floppy disks. This would be special forensic hardware though, and thus a bit expensive if you're even able to find one. This disk could then be used to revive the data on those disks so you can transfer them to some other medium. But considering the value of your data, I don't expect this to be worth the trouble.
There will be another problem, though. Are those floppies used for an MS-DOS computer or for some other operating system? They might not be using the FAT file system but some other file system which Windows won't be able to read.
Formatting those floppies and using them as extra data storage isn't practical either, since they're likely to contain 320 or 360 kilobytes of data, depending on the number of sectors on those disks. Back then, this wasn't even enough to store the MS-DOS setup! It's no surprise that the 3.5 disk quickly replaced it, since the plastic cover made them tougher and the disks could have 720 KB or even 1440 KB, depending on disk quality. (Although you could transform a 720 KB floppy to an 1440 KB floppy by drilling a hole in the right place.)
They should not be dead, though. They might be blank, but not dead. If you format them all, I guess that 95% will be reusable. But you're not interested in getting extra disk space but the content of those disks. Well, it is possible to get part of them back, depending on how much you're willing to do and pay for the data recovery. You just have to consider if it's really worth the trouble. That's just between you and the data on those disks.