> The best is defined as the optimum person to complete the required task.

What about long-term potential? Someone may solve the current task well, but will they grow in skill in order to take on larger challenges? Someone who can grow in this way may be better to keep on the same project because they will also have historical context while scaling with the project.

> The "best" is found by measuring his aptitude (tests, etc.).

Measurements for "aptitude", especially tests, are easily gamed and don't in any way guarantee actual competency.