This can only happen if the sell option is disabled such that no one can sell his bitcoin but can only buy. if that happens then we can say that a poit can reach when all the 21 milion bitcoin that is in circulation can be completely bought. but such is just possible in theory, in reality, as more people are willing to keep buy, others are always ready to sell. that is why we have a lot of buys and sells via exchages everyday. the principle behind the circulation of every medium of exchage works such that just one million can continue to circulate among people without becoming scarce as long as people are ready to spend.