Bitcoin optimists often cite its 21 million token limit. With 18.6 million Bitcoin already in circulation, it'll take close to 120 more years before the remaining 2.4 million are mined and put into circulation. The argument is that Bitcoin's fixed token count will help fight against the ongoing devaluation of the U.S. dollar as the money supply expands.
The flaw in this thesis is that Bitcoin's scarcity is nothing more than an illusion. While unlikely, community consensus could decide, at some point in the future, to increase Bitcoin's token count. Without any physical scarcity to speak of, a promise is all that keeps its token count from rising.