[dragonwriter]

> But what do you propose we do if a developer gets sick or has to travel?

There's a difference between "if a developer gets sick or has to travel" and "if a project is behind schedule". (Though if the first happens late in the project, the problems with trying to backfill with an developer with no prior connection to the project at the late date are similar to trying to stack on more staff when a project is already behind schedule, and pose similar risks of delaying rather than accelerate delivery even further than accepting the cost of the loss.)