Don't worry, be happy

Today, if you buy any amount of bitcoin, anywhere, (with $$ or whatever), someone else is selling. Every fraction of bitcoin that exists must be owned by someone (or is lost). So why are some people fretting about the 21M limit on the supply? There will always be a willing seller and willing buyer for some amount at some agreed price, right? Always. And with bitcoin transactions diced down to 8 decimal places, there will always be some small quantity available for sale. See the title.