As with all models, I'm going to start simple and make some big assumptions which some of you will disagree with.
I will disagree with both.
Assumption #1: Bitcoin will have a relatively large growth rate, which is going to decline over time as Bitcoin achieves greater market penetration and eventually reaches some sort of value equilibrium.
The growth rate can not be expected to simply decline. I expect a slow and steady, almost linear growth rate at the very beginning. After passing a certain tipping point, i'd expect a huge growth to adapt to the "masses" which would eventually fade into a steadily declining growth as you predict it.
So: linear -> exponential -> degressive? Something like that...
Assumption #2: There are risk factors which could cause Bitcoin to fail. These risk factors are roughly constant with time. They include things such as the discovery of fatal flaws in the protocol, or multinational efforts to outlaw it and shut it down.
The risk factors are definitely not constant with time.
The discovery of fatal flaws in the protocol becomes more and more unlikely over time.
The same goes for the ability of national or even multinational efforts to outlaw it.