It can't go up forever because the knowledge of that trend would make more and more people invest in them.
At some point this would cause people to build stashes of tokens instead of investing in real businesses with real production capacity.
When this trend would become widespread enough, it would eventually cause global production capacity to drop. That's right, when enough people do it, token hoarding displaces investment in businesses and factories and lowers global production capacity. This means token hoarding causes a future drop in things available to buy with these very tokens.
Eventually there will be people who want to buy real things with their stock of tokens. The tokens will be chasing fewer goods which would mean prices for stuff would rise (tokens would lose value). This could happen suddenly when people with large stockpiles of tokens notice that value is dropping and that there are tons of other tokens waiting on the sideline ready to make it drop even further.
Hoarders might rush to get rid of their stockpile all at the same time before they're worthless which would cause their fall to worthlessness. This drop would bring the tokens closer to their natural intrinsic value of zero. The cycle can then start again, such is aggregate economics.
The 1920s and 1930s suffered from this type of production drop but with gold tied currencies instead of cryptocoins. It happened to a lesser extent in 2007 when western world central banks failed to keep inflation rates high enough.
It's important for the world's sake to not let deflationary currencies become too popular.
When savings or financial promises are insufficiently tied to future production or to accumulation of real goods, there will be disappointment when many people try to exchange them for real stuff. That is true for crypto currencies as well as government currencies (that is why the system is designed to make banks invest people's money in real businesses and minimize the proportion of money that is stockpiled idly).
It's true that crypto currencies are currently not widely held enough to significantly affect the aggregate economy but speculation already keeps them volatile and the knowledge that as they get more popular, there will be more macroeconomic pressures towards volatility keeps the speculation wild and cryptocoins unstable.
Currencies that are not designed to lose value over time can not be stable. Intrinsically worthless tokens engineered to have better than market risk adjusted, liquidity adjusted, real returns compared to real productive investment will always fluctuate increasingly wildly as they get more popular.