I can understand the plateau because the amount of miners will decline and mining companies will come into existence to find bitcoins.
This will probably make the trend more based on the technology and less volatile between technology shifts. Mining will slow down considerably.
I wasn't really talking about mining. Mining secures the network (and introduces new coins into the network) the value of Bitcoin is based on what Bitcoin "can do". Note "price" may vary from value or utility but only so long. If price gets too far ahead of value/utility it will eventually crash back down. Rising value and utility is based on increased adoption and those users doing more with Bitcoins. If you double the number of people using Bitcoin and double the number of non speculative transactions they each make then the value or utility of the network will go up and with it the continued climb of price against the dollar.
However currently most Bitcoin users are relatively sophisticated when it comes to technology but as Bitcoin "taps out" that market to continue that exponential growth will require taping into markets with a lot less sophistication.
If your grandmother going to learn how to use the QT client, or deal with pywallet to manually remove tx that get stuck due to incorrect fees settings?
Is a Billionaire going to buy $1B in Bitcoins by wiring a billion dollars to companies he has never heard (MtGox) located in countries he may not be comfortable (BTC-e)?
If a a thousand "casual users" puts 1 BTC into an eWallet and the operator runs away with it will they all continue to use Bitcoin in the future like a "diehard believer" might?
Are millions (and someday tens of millions) of users going to be able to keep Bitcoins secure form hackers despite being HORRIBLY bad at security and running a Windows OS full of malware because they disable security updates that take too long?
I would guess the answer is no at least not today with the wallets, exchanges, and service providers that exist. The bitcoin infrastructure is going to have to improve, it is improving but at a much slower pace than adoption and eventually that will be the bottleneck. Growth will slow until Bitcoin can be made easy, safe, and painless for the "masses". I don't care to speculate on where or when that will happen only that there is a good chance it will eventually happen.