Mythical man month says that even if longer hours have diminishing returns, the organizational overhead from adding people to the team is worse - fewer people with longer hours is the better alternative.

Here I will quote a HN post which quotes a secondary source which analyses mythical man month

"From a business point of view, long hours by programmers are a key to profitability. Suppose that a programmer needs to spend 25 hours per week keeping current with new technology, getting coordinated with other programmers, contributing to documentation and thought leadership pieces, and comprehending the structures of the systems being extended. Under this assumption, a programmer who works 55 hours per week will produce twice as much code as one who works 40 hours per week. In The Mythical Man-Month, the only great book ever written on software engineering, Fred Brooks concludes that no software product should be designed by more than two people. He argues that a program designed by more than two people might be more complete but it will never be easy to understand because it will not be as consistent as something designed by fewer people. This means that if you want to follow the best practices of the industry in terms of design and architecture, the only way to improve speed to market is to have the same people working longer hours. Finally there is the common sense notion that the smaller the team the less management overhead. A product is going to get out the door much faster if it is built by 4 people working 70-hour weeks (180 productive programmer-hours per week, after subtracting for 25 hours of coordination and structure comprehension time) than if by 12 people working 40-hour weeks (the same net of 180 hours per week). The 12-person team will inevitably require additional managers and all-day meetings to stay coordinated."

https://news.ycombinator.com/item?id=3547965