With off-site storage the it has to updated with every transaction, or it won't be an actual backup. Cloud storage can fail, and actually has. Plus it can be hacked. And if you misplace your keys, bye-bye money. Then there's also the possibility that as technology changes those keys and wallet you stashed away 50 years ago can no longer be accessed with the new technology and software. Relying on external "vaults" run at the present time by shady businesses catering primarily to drug dealers and other criminals has its own unique problems.
Updating it regularly is also a trivial task; dropbox' software monitors a directory and as that changes, the backup is also automatically changed. Similar technology exists for Google Drive and for external USB drives.
Storage media failure: ALL forms of storage can fail, but that's not the point of an offsite backup. The odds of your hard drive failing at the same time your cloud storage goes under and vanishes at the same time your USB drive explodes at the same time your safe deposit box gets broken into are roughly the same as "getting struck by lightning
while being mauled by a polar bear". In practice, one external drive and one cloud storage (or two cloud storage) will be more than sufficient.
Being hacked: You should be far more worried about existing financial institutions being hacked than your personal dropbox being brute-forced by someone who has no idea what he's going to find there -- plus without your passphrase, those keys are STILL useless to him. Use a good passphrase when you generate your keypair, and use a strong password that you don't use on any other service and you'll be fine.
Misplacing your keys: Several solutions have already been mentioned for this, and unlike your wallet your keys don't change, so hardcopies _are_ feasible. Simply not an issue in the modern world.
Technology changing: There is always a significant overlap (think a decade or more) between 'new tech' and 'old tech' such that you will have plenty of warning that 'old tech' is going away. I can still access IDE drives from my current, modern machine trivially, and the IDE standard is _ancient_. This is not a practical concern.
As an alternate example, checkbooks are almost totally gone the way of the dodo and ATM machine use is dropping, point-of-sale swiping now being the accepted standard even at fast-food joints (i don't recommend it there, though). During all the new innovations that came with the computerization of the banking industry, it _never_ became more difficult to access money; always easier. Checks are still accepted almost everywhere, and you can even make a check out to "Cash" -- and that's just in 25 years of progress.
This sort of thing
is my field. Your concerns are, quite frankly, nothing but Chicken Littling -- the technological aspects of bitcoin are _not_ at issue here.