I am aware of that. Gold volume is not static, it increases, and the gold mine investments are also dependent on gold price. It is called sound, not because it is of static volume, but because governments can not manage the volume more than others.
Bitcoin is a bit different, because money volume will decrease as mining reward diminishes and coins are lost.
An increasing supply, as you suggest, will not take away the soundness as long as the increase is not managed at the whim of a dominator, and the new coins are distributed fairly, for example as a mining reward. So that is ok.
But it is not necessary, as it would make no difference, the future value of the money will be exactly as predictable as it will be with the bitcoin style money volume. It can never adapt to world population change, or advances in technology. It will only complicate matters (a small bit), but mostly, it will take away the absolute number one selling point of bitcoin: the fixed volume.
You nailed it in the first paragraph. Gold is sound because governments cannot control or debase it. Fixed volume is not the number one selling point of bitcoin. The number one selling point of bitcoin is that it is sound money and that it can be easily transmitted around the world and cannot be easily siezed.
Cryptocurrency can certainly adapt to world population changes and advances in technology. To have it do so in a manner that allows it to remain sound is simply a technological challenge to overcome.
Fixed volume is an effective short term marketing technique. Long term it is a significant problem.