The answer is you can borrow unlimited debt if you pin interest rates at 0% (though eventually you will lose control at the back end of the yield curve without heavy govt intervention). What happens to your currency in that process is debasement/hyperinflation.
Small tangent: I tend to borrow all the money that is offered to me at 0% effective rate, e.g., when buying a new mobile phone. I would even go as far as to borrow all money that is offered to me at sub-inflation rates. Is this rational?
Take my car insurance, for instance. I can pay it all in one chunk, up front, for $X, or I can pay two payments of $Y. But (IIRC) X < 2Y. It's not the same cost. (This is one of the ways that it's more expensive to be poor - if you don't have the money to pay for car insurance up front, it costs more.)