I'm not too sure if that's really the case. I'm not too knowledgeable in this department, but would imagine an airline would operate a service even if one sector is below the break-even point. If the other, for instance return, sector is equally low, then perhaps the airline will not operate the service. However, if combined, the two sectors are at or above the break-even point, then perhaps they will operate it.
I'm sure however it doesn't operate like this. For example, if an airline has recently introduced a new service, a prime example was the then-new SOU-ZRH EM4-operated service, and is aware that initial loads will be low, will operate the flight regardless. The airline is anticipating a rise in passenger numbers, so this risk is perhaps justifyed in the long-term.
In conclusion, I think that, if an airline has a less-than-adequately demanded service, then the concerned airline will react accordingly. If, however, the airline has recently introduced a new and thus in reality seemingly unknown service, then it will continue as it visualises an increase in passenger traffic.