Imagine a wallet (hot or not) becomes the market leader and more than half the existing bitcoins are stored there.
Then imagine it's security is breached.
If this happens at a time when bitcoin has been mass accepted and it is a dominant currency to store wealth in world-wide - then a criminal could be in possession of half the wealth in the world.....
Sounds surreal and could not happen in a FIAT system.
Any vault or storage that contains a vast amount of wealth would likely have a heavy security around it. For you to get access on the inside, you must first pass through different layers of security. Not sure what do you really want to convey in this post, but security is not bitcoin's problem, it is on the user that is maintaining the wallet to keep it secure.