Changing the reward/transaction price/whatever by a certain fraction every year doesn't imply that the price will move by the same amount, due to the original problem mentioned above: If the supply is rigid (i.e. not responding to market forces), then the price will be demand-driven, and demand can/will change arbitrarily, which means high volatility.

To maintain fixed inflation, the supply would have to be actively managed (and I assumed that's what was meant).