I think that is tough to answer because the difficulty level is determined by both processing power and competition. It responds to the hash rate of the network as a whole, so if there are more people hashing, or the miners get better processors, the difficulty will increase. I think in recent history, the difficulty increase has been primarily due to Bitcoin's popularity and increasing price, but the fancy new processers made a big dent too. I think long term, you're right. Bitcoin has to eventually reach some stable popularity and price, where I imagine the hash rate will be primarily determined by processing power. Since the protocol aims for blocks being discovered at a steady rate, the difficulty should match processing power pretty well. This is just me speaking out of my ass, of course. I think cryptocurrencies are so new that we have not much of an idea how they should best be designed and how Bitcoin will adjust over time. It does seem like the inventor(s) were planning ahead for Moore's law when he/they wrote the protocol though, for sure.