growth of bitcoin is proportional to it's population size = exponential growth.
The more people that know about bitcoin (size), the more people will find out about bitcoin (growth).
This is the nature of a viral entity. Of course, it will reach a saturation point because the population is finite, but for now, at least it's approximately exponential.
Bitcoin growth is so obviously exponential.
No problem. In this case the only parameter we should monitor - total daily volume in BTC, being circuited upon all BTC exchanges. I proof, if this volume is const, bitcoin society will freeze or collapse much before the saturation point you mentioned. I proof, time interval for such a freeze/collapse is controlled by exponential parameter: exp (t/t0), where t0 is the time, for bitcoin society to grow in e=2.71828.. times.
So, the faster is society growing, while average daily exchange volume = const, the faster society will freeze/collapse.
We observe: average daily exchange volume = const for 2 months..