1 bitcoin is 1 bitcoin, now and always. You won’t have less or more as time goes by as a side effect of the circulating supply. What will vary is its countervalue against fiat. In what direction? Nobody knows, and it will depend on many factors (demand, offer, legislation, reputation, usage, ease of use, etc.).

The circulating supply is simply the amount of bitcoins that are potentially out there, and that can be sold/bought/used at a given moment in time.
Taken to the extreme, if nobody sold their bitcoins, you’d only be able to purchase the newly mined ones, and those are reduced with around 900 new bitcoins currently mined every day, the following halving (nearly 4 years down the road) will bring that amount to roughly 450 new bitcoins per day. The overall supply will be larger as time goes by, but the effective daily circulating supply smaller. A small amount on offer, and a large demand, would rise the price like mad.
 
Of course, going to the opposite extreme, everybody could decide to sell tomorrow, and there would be a hell of a lot of circulating supply. A lot of offer, and few purchasers would drive the price down like hell.

Really, we’re in a middle ground in terms of effective circulating supply (not theoretical max. supply), and then offer and demand drive the price.