Under no circumstances should anyone rely on anything in the "cloud" for preserving digital content for decades. Companies come and go, and policies can get changed at the stroke of a pen.

Cloud storage is very useful for intermediate-term backup, but for archival storage, you need to do it yourself.

My business is preservation and restoration of various types of media, both analog and digital. Some types of media are almost ageless: 100-year-old 78 rpm records are as good as the day they were recorded, if they haven't been played. Audio tape from 1950 (which I've transferred) sounds as good as the day it was recorded, although some tape formulations, depending on storage conditions, can shed, sometimes to the point where the tape is no longer any good.

B&W film is absolutely amazing, and I have transferred negatives which are over 100 years old that look as good as the day the picture was taken. Color film is a mixed bag, and most of it fades quickly, but Kodachrome is absolutely amazing: I have some 1942 Kodachrome that has not changed at all.

Rotating magnetic media has so far generally been very reliable. I have no problem with 5 1/4 and 3 1/2 inch floppies from the 1980s (I don't have any equipment to read 8 inch floppies). Zip disks were never any good, and they have proven to be a problem even after only two decades. Hard drives have generally stored well, but the mechanics sometimes seize up. I have had problems with MFM (mid-1980s technology) drives. The problem has nothing to do with the interface (MFM) but instead with the bearings and motor. A clean room disk drive recovery company could probably recover the information, but that would be thousands of dollars.

Optical media so far has been perfect, but only with good media. I have had "bargain" media DVDs fade away after only a few years. Some laserdiscs quickly developed "laser rot," although that was a manufacturing defect that usually showed up within a few months, not years.

With quality optical media, it appears that extreme longevity is very likely: my Taiyo-Yuden DVDs still test the same now as when first burned many years ago, and accelerated aging tests seem to indicate that these discs should last for many decades, and perhaps a century or more.

So, from everything I've read, and from my own experience, optical media (DVDs in particular) seem to be the best option for long-term (decades or more) archiving. As for being able to read optical media, I think there is much, much higher probability that this standard will propagate for decades into the future, unlike disk drives where the drive interface disappears quickly (IDE is starting to disappear). The drives for optical media are like phonographs that could still play 78 rpm records for decades after the last 78 was issued. The analogy is that current "round shiny disc" players (DVD and Blu-Ray) can still read CD discs thirty years after that media (in audio form) was first made available to consumers.

Yes, it has been thirty years since the audio CD first hit the market.

Finally, the jury is definitely still out on using flash media for long-term storage, and until someone can find an accelerated aging study, I would recommend against relying on this media to store anything for longer than a decade.