What you are saying, if I understood you correctly, is that money is a method of accounting. If all money is borrowed, then money is literally worth 0, which what a unit in an accounting system is worth.

If this makes sense to you, then it'll make sense why Bitcoin is worth something. There is a circular demand to it even without debt. I'll not argue to the legitimacy of that demands (scams, phishing, laundering, etc...) but it is demand and it creates a baseline value (beyond the boom/busts it cycles through).