People have been collecting coins for ages. It has only been recently that collectors have been paying very close attention to the grades of the grades that they collect. A grade can definitely make a coin go up in value.
In the right conditions, a coin can continually go up in value. If a coin has a low supply and a high demand, it may never see a downturn. This is the reason why a lot of key dates do not go down in value. But, some key dates don't really go up in value because there is no demand for the coin. One good example is the 1970 D Kennedy Half. It only has mintage of 2.15 million, but its' demand is extremely low. But because supply and demand have to go together, this rare half dollar may never skyrocket in value. A BU specimen could be had for a mere $20 bucks.