I also asked myself about that. If there are 21 million bitcoin, is it really enough? Seeing more and more people are very curious and joining the crypto world, getting their luck and mine bitcoin, too. But then, this 21 million bitcoin makes it more special. It makes the bitcoin more interesting and challenging.
having low volume of supply comparing to the actual numbers of possible end users, bitcoin value will become huge for sure, time will let us see if how fit
those numbers if its fully enough or alts will also grow after the needs, we can't say what future will bring to this currency but being successful might give
us some hint why we need to hold and keep our coins.