Actually, I have an alternative theory to explain what happened. There is a lot of research documenting the mental benefits of exercise, especially on aspects like focus, planning, discipline, etc. These are just the sorts of aspects the author found began to wane.
Therefore, I theorize that actually it was the decrease in regular exercise that caused the change, not a vague changing habit of "completing things." After all, finishing what you set out to do can often be beneficial, but it can also be foolish when it later turns out that what was begun was unwise or low-priority.
Something doesn't seem quite right about the "always finish" theory to me, since I know many very successful people make a point of being able to stoically change priorities when necessary rather than getting mired in pointless completion of no-longer-useful goals.
The hard thing is that although we ARE ourselves, we do find ourselves reduced to theorizing as to why changes in ourselves are happening-- looking around to find a cause.
How would one test the "finishing things" theory against the "exercise increases focus" theory? No problem-- try going back to a better exercise schedule but letting things slide as needed in non-health related areas. I predict it'll be fine, suggesting it's exercise/health specifically that is affecting habits, not some sort of "getting into a bad habit of not finishing things" effect. If it's not, that suggests I'm mistaken and provides evidence in favor of the author's theory.
I'd be interested in the results.