No-one wants to spend it,
People are, believe it or not. It's just that not much people are. For now, bitcoin will just be used for "niche" transactions, until it gets higher liquidity hence higher price stability; then we will see it widely adopted without the usage of services like BitPay.
I am asking me this Q:

If everyone is holding.... and the last block is mined, supposing that on the planet earth there are more than 21mil millionaires, and each of them as a status of symbol would like to have at least 1
BTCThere will be no chance to serve them all
There will. It's just most likely going to be expensive as heck. Remember, there will always be buyers and sellers, so there will always be a chance for people to buy one full bitcoin. The question is just if they can afford it or not.
Also just to add, you. dont. need. to. buy. one. whole. bitcoin. to be able to participate and to hedge your funds.