Bitcoin cannot of course keep growing exponentially. The log ensures that its growth is decelerating on the long term to an asymptotic point.
This chart clearly illustrates the growth (uptrend) periods (shown in green) as well as the subsequent downtrend, accumulation and recovery/ distribution phases of the market cycle.
Halvings are the primary reason of those cycles as they initiate the supply shock that causes the start of the new uptrend/ bull market period.
As seen on the chart the next (3rd) Halving is scheduled for May 2020 where the new Recovery/ Distribution phase is expected to peak around $12,000. This is expected to initiate the new uptrend phase the peak of which should be considerably higher than the current All Time High, based on the parabolic pattern (see the chart below). BTC investors that follow those phases and historic patterns shouldn't wait for the next halving to buy as currently we have most likely concluded the downtrend and are at the start of the accumulation period. Roadmap