Ah, the good old limited number or "scarcity" argument. The difference between gold and bitcoin in this respect is that gold is literally limited as once it's physical stores run out, we won't have more gold but BTC on the other hand, the upper limit is hard coded.

This CAN be changed with a protocol change but the only problem would be mainstream adoption which seems unlikely.

All in all, BTC isn't truly capped to 21 mil, the upper limit can be expanded