I'm not very sure the exact meaning of this issue after reading your outlines. Initially, I thought it was about the fairness of promotion to all employees. But from your outlines, it seems not what I had thought. My outline is as follows.
1/ Granting all employees the opportunity to move into management-level position is the basic obligation that all companies should provide to its employees. Under our social security law, all companies should work against discrimination against sex, race, and religion. ....Moreover, the action of granting fair opportunity is consistent with our democratic ethics of opportunity equality. ...
2/ By providing equal oppotunity of promotion, emlpoyees will feel motivated and deserved of contribution.... In addition, it will help form a homoniousness and collaboration between employees.
3/ It would also contribute to a better reputation for the company. For example, the "Bench Plan" in IBM