The problem isn't debt, it's saving - if savings keep building up without being spent you have to keep creating new money to replace them in the circulating economy, and you're potentially building up trouble for the future if the savings begin to be spent again at some point. It's similar to the problem companies have with loyalty points - if people don't redeem them and they don't expire companies could eventually find that they have a huge overhang which could potentially wipe out their profits for a long period if they were all spent at once. (Maybe I've missed it but I don't think any company publishes their outstanding exposure to loyalty points ...)