The lifespan of TLC SSDs (AKA the affordable ones) isn't terribly long, about 15 years. Maybe that's long enough for your situation, maybe not. MLC drives should last about 60 years which should be long enough for any individual but they're still extremely expensive and if you're wanting to pass something on to your grandchildren then that won't cut it.
If you want long term storage that will always be readable go with tape. The tapes themselves barely have moving parts so if something breaks you can just move the actual tape to a new reel. If the reader breaks you can just buy a new one. Of course, this is going to be the most expensive option, the readers start at $3000 for the entry level ones. This makes sense for businesses since for the price of a single 1TB drive you can pick up dozens of terabytes worth of tapes, but for an individual you can buy 90TB worth of HDDs for massive redundancy for just what the reader would cost which could be 9 redundant copies of 10TB worth of data.
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If you aren't writing to it often, like a bi-weekly backup it will either die within the first week like most faulty SSDs do, or outlast a HDD by a long time. (of course, assuming propor trim settings and such.)
I'd suggest getting a Intel 730 or Samsung 840/850 pro in raid 1 if you want 20+ year storage.
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