I'm not sure, exactly - most of the discussions about whether Fiber would be viable happened long before I was on the team, and in any case I wasn't privy to them. Here is Milo Medin's (Google Fiber's "founder") testimony before the House Oversight Committee on regulatory issues in ISPs:

https://oversight.house.gov/wp-content/uploads/2012/01/Testi...

(From Apr 2011, over a year before Google Fiber actually launched.)