Apart from what everyone else has said... which is that if someone hoards and drives the scarcity up, someone will eventually offer enough for someone else to sell...

The other thing to keep in mind is that if bitcoin became even more scarce, they could just add more subdivisions, smaller than the current satoshi.. you could call them finney's, or if it was really insignificant, wrights

So you can keep sub-dividing bit coin and each smaller portion will just get worth more as scarcity increases.

The bigger issue is attrition.. the number of people that lose access to their bitcoin forever... that will make bitcoin more scarce than people hoarding it.