One can have a discussion without thinking it is going to happen.  It is useful to provide an upper bound on value.  One Bitcoin for example will never be worth $10 trillion USD when measured in 2010 dollars (adjusting for inflation).  One the other hand the bitcoin money supply is currently only worth about 0.006% of global money supply.  So while Bitcoin will probably not replace ANY currency (much less all of them) if it carved out a niche where it was used by say 1% of the population it shows the upside potential is significant.