The trendline is recalculated every time a datapoint is added. This changes its starting point and slope (kx+b).
As the recalculation happens so frequently, and adds only less than 2% of new data, the changes are very small.
The predictive power of the trendline is under constant evaluation. If the price fails to go below the trendline before enough time has passed (perhaps because of a new bubble), it is already more expensive to buy in absolute terms, even if the price later tumbles well below the trend. We are talking at most a few months of waiting that could still be profitable.
On the other hand, even after such a black swan event as the rise from 0.005 to 32 in less than a year in 2010-2011, the trendline was duly visited before uptrend resumed, and the low predicted with considerable accuracy. This gave me an entry point of about $3, which of course is much worse than in the beginning of that year, but very good compared to May-Oct
This is guesswork, but I give greater odds (perhaps 65-35) that we continue the correction now (as an alternative to reinflating the bubble). Even if the bubble reinflates, it will pop so quickly that buying at <1000 is a very reasonable chance.