That agent must never have read a historical on the bestseller list, or done any research at all on writers. It took Ken Follett three years and three months to write Pillars of the Earth, not ten years.
And few of us are going to write 900 page historicals like Pillars until we make a name for ourselves, and probably not even then.
Pillars was also originally published in Germany, as Follet's publisher didn't believe that people wanted to read a novel about building a cathedral. It was only after it was a run-away hit in Germany that it was published in the UK.