"At least the labourer's salary will increase which would be fair and his manager's will decrease."

That's one of the reasons for maintaining the sponsorship system, unfortunately.

Aside from giving business owners (ie nationals) some security that wages will be kept under control, it is also a good means for the central bank to keep a lid on inflation.

This, I'm told, is one of the reasons why it has been so difficult to switch jobs, even with an NOC. The Ministry of Labor recognises that generally people move jobs to earn more money, so allowing people to switch jobs would contribute, however marginally, to a potential wages spiral.

Keeping bargaining power in the hands of the employer reduces the potential for wages, and therefore inflation, to get out (more) out of control.