A common misconception is that while Bitcoin may be deflationary in that the rate of supply creation is decreasing, it does not in any way guarantee the value of the coin. Inflation and deflation of currencies are also demand based. If demand for bitcoin falls faster than the decreased rate in which coins are issued, the value must go down.

As with most financial assets with high liquidity, there is no such thing as "guaranteed to become more valuable". If it were, it would already be at that price.