.... if the aim is to treat all sources of employment income the same (i.e. fairness), then obviously those benefiting at the moment will be worse off.

The question is whether we want to allow some people to pay less on the same amount of gross income earned at the end of the day. Is it fair that a doctor or software engineer that earns $100,000 gets to pay less in tax than an office manager or other employee earning the same amount?