Do you honestly believe he planned so far ahead?
Its no secret that most if not all Mangas progress as time goes on
It's no secret you plan the major and crucial parts of the story first and then just fill in the details. Kishi already has planned all the important and crucial details, while only filling in the less important details as the manga goes on. Hashirama is powerful, I admit, he's one of the most powerful in the series, but that doesn't mean he can't be beat, just like it doesn't mean that Hiruzen can't be beat if he's stronger than Hashirama. I also think that Hashirama would've wanted Hiruzen to be stronger than himself, otherwise he would be disappointed at training a Hokage weaker than himself (Hashirama) to protect the village he built. Additionally, Hashirama wasn't at all surprised that Hiruzen defeated him when the two fought and neither was Tobirama.
Think of it like this: fire beats wind, wind beats earth, earth beats lightning, lightning beats water, water beats fire. Now tell me, which one's the strongest one? None, they all have their strengths and weaknesses. If Naruto is proven to be like DBZ where there is only one single strongest character who can't be beaten, Rikudou Sennin is the exception because of having the Juubi, I'm quitting Naruto and I'll hate it for the rest of my life. The one thing that attracted me to Naruto the most is that there is no single strongest character, that the fights all depend on who's smarter, has more skill, has more experience, and can get the most out of his or her jutsu.
We can always disagree, but if you wish to have a discussion on Hiruzen vs Hashirama, I see no problem in defending my position that Hiruzen can defeat Hashirama.