Nobody truly owns any bitcoins..because the ownership is only established when you are able to apply a correct private key in a "send" tx (via hardware device or something else), albeit validation of the private key (at some other point in time) is possible. Before that, it is all allegedly. This is in principle, of course.
This is like saying that your house is not truly yours, until you put the key in the lock and open the door...
While I can see the philosophical side of your argument, I don't think it serves a purpose, really... My keys give me guaranteed access to my BTC, period. It has never failed me and never will, because it is guaranteed by code, math & science. Should a technical glitch occur (a bug in the code, perhaps, or a mempool overload), that prevents me from accessing my coins, this will likely be a global thing affecting a wide range of key owners, and should be fixable. To the best of my knowledge, throughout Bitcoin's history, there has never been a case of someone's keys not working.
Losing your keys is not relevant to the discussion. In the same manner, the possibility that you could cross a road and be killed by a passing car means that you're not really alive right now. I can see this turning religious fairly quickly...