While that is true, you would be paying a 'one pizza loan' back with the number of Bitcoins that would now buy 60 pizzas. It is the same deal as if you were doing dollar conversions. A mortgage, 10 years later, will be the amount of BTC that could now buy two of the very same home you borrowed for. See how borrowing is just a bad deal with deflation? People would start living within their means and spending/GDP would plummet. The current power structure would be upended (financially speaking) and a new system of economics/commerce would emerge.
It really depends on how stable Bitcoin becomes as a world currency. The whole 1BTC now == 60BTC later thing might look right now, but at some point I'd hope that the BTC:USD price would stabilize, either because BTC becomes big enough that the daily purchases and sales don't affect the price that much (more likely) or because governments drop their own currency in favour of BTC so there is no continual rebasing required (very unlikely but wouldn't it be nice?)