No. Accounting for everything is not the same as doing everything. You can tell me it will take a week to implement a feature and it may take you half an hour to come up with that estimate. It doesn't mean it took you half an hour to implement the feature!
Then I wouldn't be accounting for everything, and my estimate is most likely wrong.
I might not account for the cat tripping over my network cable.
I might not account for a server unexpectedly crashing.
I might not account for the office being too noisy to think in on Tuesday.
I might not account for Amazon having EC2 issues
I might not account for a DDoS attack on my DNS provider.
All of those things impact the time it would take, and the only way to fully account for everything that could happen would be to actually complete the project. Of course, if the project was identical to one I've done before, I could get a lot closer to an accurate estimate, with fewer real unknowns, but in 20 years of software, I've never had to create an identical solution. Because if it was an identical problem, I'd just use the previously developed software.
