But that applies just as well even if you don't already own the bitcoin (you could just buy bitcoin instead of the other thing). Why does anyone buy anything that's not bitcoin? Because they want those things more than they want bitcoin. That doesn't stop being true just because you can pay for things in bitcoin.

Identical reasoning applies if you replace "bitcoin" with anything else that you think is going to go up in value over time. And people still buy things. The sky isn't going to fall down!