You see every time people speak on those issues they fail to consider the fact anything can be stolen. Burgler can take your computer by breaking into your house, your traditional cash money can be taken from you, your car can be hijacked. The whole ownership concept presumes that your thing can cease being yours at some point (it can be done with or without your consent). Id doe not matter if it is truckload of gold or virtual coin. If value is transferable - it can be transferred, no matter of its quantity. So it all boils down to how do you make the effort of stealing  cost more then what the thing being stolen. For example effective defence would be to hide 10$ behind 5$ lock that require 1000$ tool to break it. I.e. thief need to invest times more to steal times less.

Another fundamental rule is that anything can be taken off of you provided enough efforts were invested. So if I have infinite resources I can theoretically take appart any lock atom by atom - it just not very practical.

Cryptocurrency is interesting in that context because with current technological progress there is no way to BREAK your key. So the only weakness is your own head. So "ownership" of coins and safety is your sole responsibility. Basically you get the perfect lock, but you need to design the door yourself. If your door have a hole - your lock won't save you.