It's not particularly surprising: credit relies on trust, which in turns can rely on either
1. close social relationship between participants (the traditional society model) or
2. the existence of a coercive force you can call to help you get your credit back (the state).
MMORPG have 1. inside clans and guilds, and as such lending between clan members is commonplace, but since there's no 2. the debt-based economy can never arise at game scale.
What doe the "existence of a coercive force you can call to help you get your credit back," would look like in a MMO, and how you could gamify that mechanic. Maybe something like a credit score and bounty hunters? smart contracts? (not crypto based.) Fun to think about.
People are already complaining about “having to work” to cope with the gold sinks in MMOs, I don't think many would find it fun to be tracked by repo men in game.
This covenant system created a deep and engaging playerbase. I've not seen any other game pull such a system off, but it was fun. If the risks and rewards are right, it can work.