The more you extrapolate the more frequently you will need to adjust your future predictions because of errors in your initial measurements. This goes for any kind of extrapolation (for instance: plotting a course on a map), but it goes even more for extrapolating the future from limited evidence present today. Your 'best guess' might be off by many orders of magnitude if the evidence you have today is only loosely related to the future in terms of importance and where evidence may not be nearly as independent in nature as you currently perceive.

This can lead to your best guess based on available evidence being about as good at predicting the future as randomness in spite of all the apparent effort at making the predictions mathematically sound.