In my experience many excellent hackers don't even try to make the educated guess about how much change to account for. The Hacker assumes that the system must handle various far-off scenarios which gives them an excuse to play with [insert tech here]. Or the Hacker assumes that "the business" has thought of the best strategy for dealing with various technology issues that might occur (as if non-technical types could really do that), which gives them the ability to say they were just doing what they were told.

The industry conditions Hackers to avoid thinking strategically, at least in part. The Junior Hacker is browbeaten for not thinking of every possible scenario when things go wrong, however unlikely. The Junior Hacker is rewarded for following the directions of their technical supervisors to the letter without thinking. It shouldn't surprise anyone that by the time the Hacker is promoted to Senior Hacker, they have little experience designing and developing software that will ship quickly, not be a maintenance burden, and can adapt to success if it comes.