[...]
What I yet don't understand is: What grants you ownership rights, if not the private key? Car ownership is granted by the law, and same is true for almost every asset. For that reason, I'm not convinced that taking the keys of a car is a good analogy to taking the private key of a bitcoin wallet, and no one should be. It might be a good comparison morally-wise, but not beyond that.
Since it's a moral question, not a legal one, it doesn't matter whether there are documents (like in the car analogy) that grant ownership rights, because these documents just formally state the previous owner's intent of transferring ownership.
If I understand o_e_l_e_o correctly - and I think this is how I would see it now, too - a credible way that
something's previous owner
proves their intent to transfer ownership, is a moral requirement for
taking something, as it
morally grants you ownership right.
As I said before though,
technically, you do own Bitcoin the moment you have the corresponding private keys - that's why we always say:
not your keys, not your coins.
I do get where you're coming from, though. Why is it
not your keys, not your coins with cars, too? Why don't I own a rental car as soon as they hand me the keys? Or if I choose someone to custody my car keys for me?