Why does your reasoning fail at such a simple level? You're talking about some Satoshi's system, banking system, liabilities and god knows what else. I'm not interested in those things. I just have one simple question for you.
Please bear with me as I explain slowly. I had "something" associated with my address. I went to an online exchange, I exchanged it for fiat currency (for $ 44k at current exchange rates), and now I don't have that "something" anymore. All I have left is just a address. By definition, this process is called sales. My question to you, what did I sell?
My answer to you is: nothing. You sold nothing.
The answer makes no logical sense, however. Why can I sell "nothing" but you can't?
Take this analogy. I write down my name and "1 Zeus" next to it. I now have "something" associated with my name. I go to my neighbor, I exchange it for fiat currency (for $ 44k) by writing down his name and "1 Zeus", and by writing down my name and "0 Zeus" next to it. Now I don't have that "something" anymore. All I have left is just my name. By definition, this process is called sales. My question to you, what did I sell?
Your analogy has one major flaw. You can write "1 Zeus" next to your name again (after the sale), and it will be indistinguishable from the original record, while I can't simply write 1 BTC next to my address. Do you see the difference?