It will increase in value forever because it is limited while the demand for use now and in the future is increasing. No predictable value so it's perfectly possible.
If we suppose that Bitcoin will take the place of fiat currencies in the future (meaning everybody in the world is paid in btc, and use btc as a currency), then usd will worth hardly anything and bitcoin will worth a lot compared to $.
So yes I think bitcoin price can raise for quite a long time, not only because bitcoin has limited number of coins (and number of btc really used will go lower once every coin has been mined because everyday some coins are lost), but also because fiat currencies we compare bitcoin to will decrease in value.