Offline storage doesn't protect you perfectly, you can lose your keys no harder than losing your car keys. You can come up with a thousand ways to protect your coins but none are fail safe. guess where i put my cold storage keys, a safe deposit box. So i trust the bank more than my house or anything else. Funny how we come full circle. If I put my offline wallet in a bank, why not just trust a bank with the keys themselves?
You can put your *encrypted* offline wallet in several bank deposits (copies). The encryption makes that the bank, or any law enforcement, or burglars that break into the bank, or whoever opens your bank deposit, cannot do anything with it. The fact that you can have several copies around the world in different bank deposits means that you cannot loose them (even a partial nuclear war would, hopefully, not bomb out all the banks where you have a copy in a bank).
The encryption phrase, you keep it in your head. Eventually, you encode it in a line in your will.
As such, you have to trust the bank(s) much less than you would need to trust them for handling your money directly.
You keep one or two copies in your home (also encrypted of course) in order for you to have easy access. The day your house burns down, or that burglars steal your USB stick, or whatever, you simply go to one of the banks where you put a copy, and you make a copy again.
That's a nice start, but there are several problems.
First of all, passing the secret to your family members in case you are deceased unexpectedly (who expects this, right)? Trusting a key to a lawyer is not very wise.
Second of all, couple of guys come into your house, politely hold a gun to your head and ask you to decrypt that little wallet you so conveniently have in your house. Now all of your security goes down the drain.
Much better solution would be to have a multisig wallet that would require secrets from, say, 3 out of 5 trusted people to sign. This way robbing you becomes impossible, your death does not block your family from accessing funds, and you can store the wallet anywhere you want, even on dropbox. Exact numbers depend on your family configuration, surely if you don't have people whom you can trust fully this also would not work - but for such cases, I'm sure there will be trusted third-party arbitrages soon who could hold your extra key and who will be able to sort it out in case you split up with your wife.
But the one thing which is clear now - only when you yourself can not access your funds immediately, they are safe. For spending money, use single signature wallets, but for any substantial amounts it's just asking for trouble.
