If you connect that drive back to the NAS, do you see the files? If you can, then the drive was probably formatted to a file system that your iMac can’t read.

Is there any reason they won’t work after that amount of time?

Your first problem isn’t how long these drives will work but how to check if they still work 50 years from now. Do you think computers in 2073 will still have the connectors being used today (e.g. M.2, SATA, SAS, U.2, U.3, EDSFF, etc)?

Take PATA HDDs for example. That storage interface began disappearing from motherboards in 2007. HDDs using that interface stopped production about 10 years ago. As years go by, even adaptors for that interface will become rarer. If you asked this question 20 years ago but instead of SSDs you stored new PATA HDDs, do you think you’ll be able to check if they’re still working 30 years from now?

To workaround the connector problem, you’ll also have to store equipment that will work with the interface for those SSDs. Maybe a motherboard, an adaptor, even a power supply. How long would those extra hardware you have to store will last?

The PM987 would have a 21.024 PBW endurance rating compared to the 2.4 PBW of the 990 Pro. Not sure if it has PLP though but it looks like it has the capacitors for that feature based on the pic from TPU.