WRONG. BTC has not been going exponential - if you ignore the last week of trading. It has been closer to linear.

If it ever does start to go exponential, like it did in the last week, THEN you have reason to worry. And you have a guarantee that the price rises will stop the exponential trend and prices drop back to earth and resume linear growth best case.

Exponential curves are not sustainable. Linear curves are, within some limits.