I propose that the number of bitcoins in active circulation will ultimately reach zero.  This is not going to happen anytime soon, but it will happen eventually - might be 1 year, 100 years or might be 1000 years I don't know.  The reason I believe it will happen is a combination of three factors.

1) It is impossible to access the coins of a properly secured wallet
2) There will only ever be a finite number of bitcoins (21M)
3) Humans do not live indefinitely

This means that as people who hold bitcoins in a completely secured manner pass away (i.e. no one else has access to their coins) or simply lose their wallets, their coins are removed from active circulation or effectively destroyed.  I'm pretty sure there is a considerable number of coins which were mined in the beginning which are now lost (I know I personally had some when they were being exchanged for about 25 cents which I lost)

Compare this to traditional currencies, where my money is stored in a bank, stuffed under my mattress or stored somewhere that noone can find.  There is a few things that can happen when I die, either
a) The executor for my estate will find the money wherever it is stashed and gain control of it (if it is in the bank or mattress). 
b) If an executor doesn't find it then it will go to the government if it is accessible (e.g. bank account with no will/executor to claim the money).
c) If the money does get completely lost then the money will ultimately get replaced by the printing of new money.

That's why a traditional currency is not susceptible to this while bitcoin is.  Because of this, at some point the scarcity of bitcoin will reach such a point that they loose all their value as so few people will have them that they have no value for exchange.